The end of trial by ambush

Picture the climax of every legal thriller you have ever watched. The witness nobody expected strides through the courtroom doors. A surprise document lands on the clerk's desk. Opposing counsel leaps up; the judge bangs the gavel; the case turns on a revelation no one saw coming. It is wonderful theater. It is also, as a model of justice, a catastrophe—because a trial is supposed to test which side is right, not which side hid the ball more cleverly until the moment of maximum damage.

For most of the long history of litigation, surprise was a legitimate weapon. A shrewd lawyer might sit on a smoking-gun document, a key eyewitness, or a devastating expert opinion and spring it on an unprepared opponent who had no chance to investigate, depose, or rebut. The modern Federal Rules of Civil Procedure were built to dismantle that game, and Rule 37(c)(1) is the enforcement mechanism at the heart of the design. The rule embodies a simple, ruthless bargain: the Rules require you to lay your evidence and witnesses on the table during discovery, and if you don't, you generally forfeit the right to use them. Information you should have disclosed but didn't cannot be deployed at trial, on a summary judgment motion, or at a hearing. The surprise witness never reaches the stand. The withheld document never reaches the jury. The undisclosed expert opinion is excluded. Preclusion is the price of nondisclosure, and the price is steep.

That makes Rule 37(c)(1) one of the most consequential procedural tools in federal civil litigation—and one of the most underestimated by lawyers who think of "sanctions" as something that happens only after a motion to compel and a judicial scolding. A well-aimed preclusion motion can gut an opponent's case in a single stroke: knocking out the only evidence supporting a damages theory, the only witness to a critical event, the expert without whom a claim cannot legally be proven. Conversely, a party caught having failed to disclose can watch its case collapse not on the merits but on a discovery default that, with a calendar entry and a moment's diligence, would never have happened. Understanding exactly how the rule operates—what it covers, when it bites, how to escape it, and how to wield it—is therefore essential for any litigator and genuinely useful for any business owner trying to understand why their lawyer treats discovery deadlines as sacred.

This guide focuses on how the rule is applied in the federal courts, with particular attention to the Second Circuit and the Eastern District of New York (EDNY), whose controlling articulation comes from Patterson v. Balsamico, 440 F.3d 104 (2d Cir. 2006). But because Rule 37(c)(1) is a national rule and the multi-factor analysis it invites varies in interesting ways across the circuits, we also map the Fourth Circuit's influential framework in Southern States Rack & Fixture, Inc. v. Sherwin-Williams Co., 318 F.3d 592 (4th Cir. 2003), and the Tenth Circuit's in Woodworker's Supply, Inc. v. Principal Mutual Life Insurance Co., 170 F.3d 985 (10th Cir. 1999). Preclusion frequently determines what record a court actually has in front of it when it decides a case—including, in a trademark dispute, the evidence available to prove or disprove the confusion factors we analyze in our companion piece on the Polaroid factors on summary judgment in the Second Circuit. What you cannot disclose, you usually cannot use—and what your opponent cannot use, the court will never see.

The duties the rule enforces: Rule 26(a) and Rule 26(e)

Rule 37(c)(1) is not a roving license to punish discovery sins in general. It is bolted to two specific sets of obligations under Rule 26, and the scope of the rule's preclusion power matches the scope of those duties precisely. You cannot understand the sanction without first understanding the duties it backstops, so start there.

The first is Rule 26(a), which requires parties to make several categories of disclosure without waiting to be asked—an architecture that distinguishes the federal system from older "request-and-response" regimes. Initial disclosures under Rule 26(a)(1) require each party, early in the case (ordinarily within fourteen days of the Rule 26(f) conference), to identify the witnesses and documents it may use to support its claims or defenses, to provide a computation of each category of damages it claims, and to make available any insurance agreement that may satisfy a judgment. Expert disclosures under Rule 26(a)(2) require parties to identify their expert witnesses and, for retained experts, to serve detailed written reports setting out the opinions, the bases and reasons for them, the facts or data considered, the witness's qualifications, the witness's prior testimony, and the compensation. Pretrial disclosures under Rule 26(a)(3) require parties, ordinarily at least thirty days before trial, to identify the witnesses they expect to call and the exhibits they expect to offer. These are affirmative duties. The Rules place the burden on each side to show its cards at the appointed times—not when an opponent thinks to ask, but as a matter of course. (We unpack the mechanics of these obligations in a practical discovery refresher: mastering the tools, rules, and pitfalls of federal civil litigation.)

The second obligation is Rule 26(e), the duty to supplement, and this is the one that ambushes careful lawyers as often as careless ones. Rule 26(e) requires a party to supplement or correct its disclosures and its discovery responses—including answers to interrogatories, responses to requests for production, and responses to requests for admission—in a timely manner if it learns that a prior response was in some material respect incomplete or incorrect, and if the corrective information has not otherwise been made known to the other parties during discovery or in writing. The duty is continuing: it does not expire when the initial deadline passes or when discovery closes. A party that answers an interrogatory incompletely and later acquires the missing information must supplement. A party whose production omitted a responsive document must produce it once the omission surfaces. An expert whose report needs correction must have that correction served. This supplementation duty is what stretches Rule 37(c)(1)'s reach beyond the formal Rule 26(a) disclosures and into the substance of ordinary, request-driven discovery—a point that matters enormously when one side tries to spring at trial something it quietly withheld in its written responses. For a deeper treatment of how those written responses are crafted and policed, see mastering document discovery: a comprehensive guide to objecting and responding to requests for production and mastering interrogatories: a comprehensive guide to objecting and responding in civil litigation.

The link between these duties and the sanction is direct and unforgiving. Rule 37(c)(1) applies precisely when a party "fails to provide information or identify a witness as required by Rule 26(a) or (e)." If the obligation to disclose or supplement was triggered and the party did not comply, the preclusion machinery is engaged—no order, no warning, no second chance required. This is also why a party cannot launder a discovery default by simply staying quiet: the duty to disclose and to supplement is affirmative, and silence where the Rules required speech is itself the violation. The rule punishes the omission, not merely the refusal to comply with an order.

What the rule actually says—and does

The operative language is compact and severe. If a party fails to provide information or identify a witness as required by Rule 26(a) or (e), the rule provides that "the party is not allowed to use that information or witness to supply evidence on a motion, at a hearing, or at a trial, unless the failure was substantially justified or is harmless." The rule then adds, in subparts (A) through (C), a menu of additional or alternative sanctions—payment of the reasonable expenses (including attorney's fees) caused by the failure; the option of informing the jury of the party's failure; and the imposition of "other appropriate sanctions," including any of the orders listed in Rule 37(b)(2)(A)(i)–(vi)—available "on motion and after giving an opportunity to be heard."

Three features of this text do most of the work in practice.

First, the breadth of the preclusion. The barred party may not use the undisclosed information or witness "on a motion, at a hearing, or at a trial." Preclusion is therefore not just a trial sanction; it operates at summary judgment too. A party that failed to disclose a document, a witness, or a computation of damages cannot rely on that evidence to defeat—or to win—a summary judgment motion. Because the summary judgment record is so often outcome-determinative, this means preclusion can decide a case months before any trial date. A defendant who never produced the contract it now wants to wave at the court, or a plaintiff who never disclosed the damages computation it now seeks to prove, may find the missing evidence simply unavailable when the dispositive motion is briefed.

Second, the self-executing character of the sanction. The Advisory Committee that added this provision in the 1993 amendments described the preclusion remedy as "self-executing" and "automatic," designed to provide "a strong inducement for disclosure" of the material the Rules require. The significance is structural. Unlike Rule 37(b), which authorizes sanctions only after a party violates a court order compelling discovery—and thus presupposes a prior motion to compel and a prior order—the preclusion sanction of Rule 37(c)(1) requires neither. The violation of the disclosure duty itself triggers the consequence. The Ninth Circuit put the point memorably in Yeti by Molly, Ltd. v. Deckers Outdoor Corp., 259 F.3d 1101, 1106 (9th Cir. 2001), describing Rule 37(c)(1) as a "self-executing, automatic sanction to provide a strong inducement for disclosure." A litigator who assumes that an opponent's nondisclosure is harmless because "they never got an order compelling it" has misread the rule entirely.

Third, the two escape hatches. The preclusion is not absolute: it does not apply if the failure "was substantially justified or is harmless." These two exceptions are the battleground on virtually every contested preclusion motion, and—as the next section explains and as the case law uniformly holds—the burden of establishing them falls on the party that failed to disclose, not on the party seeking preclusion.

The two escape hatches: substantial justification and harmlessness

A party facing preclusion can avoid it by showing that its failure to disclose was either substantially justified or harmless. These are independent exceptions—either one alone suffices—and, critically, the burden of proving them rests on the non-disclosing party. Courts are emphatic about this allocation. As Yeti by Molly explained, the rule places the burden on the party facing sanctions to prove harmlessness; the implicated party "has the burden of showing that a failure to disclose was substantially justified or harmless." 259 F.3d at 1107. The practical consequence is that once the moving party shows that disclosure was required under Rule 26(a) or (e) and did not happen, the spotlight swings to the defaulting party, which must come forward and excuse or justify its own conduct. This is the opposite of the ordinary intuition that the party seeking relief carries the laboring oar, and it is one of the most important strategic features of the rule.

Substantial justification asks whether there was a genuine, reasonable basis for the failure to disclose. Courts borrow the Supreme Court's formulation from a different statutory context (fee-shifting under the Equal Access to Justice Act): conduct is substantially justified when it is "justified to a degree that could satisfy a reasonable person," or, as the Court put it, when it has "a reasonable basis both in law and fact." Pierce v. Underwood, 487 U.S. 552, 565 (1988). Translated into the disclosure setting, substantial justification might exist where the scope of the obligation was genuinely ambiguous, where the information truly did not exist or was not within the party's possession, custody, or control at the relevant time, or where a reasonable lawyer could have read the governing rule or request not to require the disclosure. What does not count as substantial justification is the ordinary stuff of litigation neglect: oversight, a misreading of a plain obligation, strategic delay, or a simple failure to calendar a deadline. A bare assertion that the omission was inadvertent rarely carries the day; courts have heard "it slipped through the cracks" too many times to be moved by it.

Harmlessness asks a different and often more decisive question: regardless of why the disclosure failed, did the failure actually prejudice the opposing party? A failure is harmless when the omission caused no real detriment—where, for instance, the information was already known to the other side through other discovery, where it was disclosed in a different form or document, where the opposing party deposed the witness anyway, or where the lapse was a genuine technicality that deprived no one of a meaningful chance to prepare. The touchstone is prejudice. If the opposing party had, in substance, what it needed to meet the evidence, the failure may be excused even though it was a technical violation. Conversely, where the late or absent disclosure deprived the opponent of the chance to investigate, depose, retain a responsive expert, or shape its trial strategy, the failure is not harmless—and "the more important the information, the less likely its late disclosure can be brushed aside as harmless." Some courts also fold into the harmlessness inquiry a concern about disruption of trial: a late disclosure that would force a continuance or derail an imminent trial is, almost by definition, not harmless.

The interplay of these two exceptions is worth internalizing because it explains who wins and who loses. A party with a good reason for the lapse can prevail on substantial justification even if there was some prejudice. A party with no good reason can still prevail on harmlessness if the lapse genuinely hurt no one. But a party with neither a good reason nor an absence of prejudice is squarely in the rule's crosshairs—and that, candidly, describes the typical preclusion target: a litigant who forgot, or gambled, and got caught.

Mandatory in form, discretionary in practice

Here lies one of the most important—and most misunderstood—nuances in the entire doctrine. Read literally, Rule 37(c)(1) sounds mandatory: the party "is not allowed to use" the undisclosed evidence. And the Advisory Committee called the sanction "automatic." Yet the Second Circuit, like its sister circuits, has not applied preclusion as a rigid, mechanical guillotine in every case. It treats preclusion as a discretionary remedy—indeed, a "harsh" one—to be imposed after weighing the particular circumstances, and it has cautioned that the most severe form of preclusion (one that effectively decides the case) should be reserved for relatively serious or willful violations. The Second Circuit has said as much repeatedly: preclusion is a "drastic remedy" that should not be imposed lightly.

How can a single rule be both "automatic" and "discretionary"? The reconciliation is elegant once you see it. The availability of preclusion follows automatically from a violation that is neither substantially justified nor harmless. But the exercise of that remedy—particularly the choice between outright preclusion and some lesser sanction—remains committed to the district court's sound discretion. The court does not ask only "was there a violation?" It asks "what is the appropriate response to this violation, given its seriousness, the importance of the evidence, the prejudice, and the alternatives short of exclusion?" That second question is where the structured, multi-factor analysis enters—in the Second Circuit, the framework most often associated with Patterson v. Balsamico.

This discretionary character is simultaneously good news and bad news for both sides, which is exactly why both sides must brief it. For the party seeking preclusion, discretion means the remedy is not guaranteed even after a clean showing of violation; the court may pick a lesser sanction or excuse the lapse outright. For the defaulting party, discretion means a violation is not necessarily fatal; a persuasive showing on the discretionary factors—especially a prompt cure—may save the evidence or soften the blow. Both sides must therefore argue not just whether a violation occurred, but whether preclusion is the right answer to it.

The Patterson / Softel factors in the Second Circuit

In Patterson v. Balsamico, 440 F.3d 104 (2d Cir. 2006), the Second Circuit set out the factors a district court weighs in deciding whether to exercise its discretion to preclude evidence under Rule 37(c)(1). The factors did not originate in Patterson; the court drew them from its earlier decision in Softel, Inc. v. Dragon Medical & Scientific Communications, Inc., 118 F.3d 955 (2d Cir. 1997), and they are accordingly sometimes called the Softel factors. Patterson is, however, the decision EDNY and Second Circuit courts most often cite as the controlling articulation, so we use its name as shorthand for a framework that the two cases share.

A court considers four factors: (1) the party's explanation for its failure to disclose; (2) the importance of the precluded evidence; (3) the prejudice suffered by the opposing party as a result of having to confront the new evidence; and (4) the possibility of a continuance to cure that prejudice. Each repays close attention, because the way the four interact—not any one in isolation—determines outcomes.

The first factor—the explanation for the failure—overlaps heavily with the substantial-justification inquiry and tends to set the tone for everything that follows. A party with a credible, good-faith reason (genuine ambiguity in the obligation, late-discovered information, an honest mistake promptly corrected) stands on far firmer ground than one whose explanation is silence, neglect, or transparent strategy. Courts are notably unmoved by explanations that boil down to "we forgot" or "we didn't think we had to," and they are openly hostile to explanations hinting at deliberate withholding. A weak or absent explanation pushes hard toward preclusion and colors the court's reading of the remaining factors.

The second factor—the importance of the evidence—is genuinely double-edged, and litigants routinely misjudge which way it cuts. The intuitive assumption is that the more important the evidence, the more reluctant a court should be to exclude it—exclusion of crucial proof feels like a punishment that swallows the merits. But the Second Circuit has expressly recognized the opposite pull: the more important the evidence, the more crucial it was for the disclosing party to have surfaced it on time, and the greater the prejudice from springing it late. Importance is therefore not a one-way ticket to admission. It can heighten both the equities against preclusion (the party loses something significant) and the equities for it (the opponent was sandbagged on something significant). A party that walks into court assuming the centrality of its evidence will save it has misunderstood the factor.

The third factor—prejudice to the opposing party—is frequently decisive and tracks the harmlessness inquiry almost exactly. The question is whether the late or absent disclosure deprived the opponent of a fair opportunity to meet the evidence: to investigate it, depose the relevant witnesses, retain a rebuttal expert, or recalibrate its strategy. Prejudice is greatest when the disclosure surfaces late—on the eve of trial or after discovery has closed—leaving no realistic way to respond. It is least when the opponent already had the substance of the information or still has ample runway to react. A movant seeking preclusion should document the prejudice concretely and specifically: what it would have done differently with timely disclosure, what investigation is now foreclosed, what the surprise would cost to cure. Vague invocations of "prejudice" persuade no one; a precise account of foreclosed depositions and missed rebuttal opportunities persuades.

The fourth factor—the possibility of a continuance—asks whether simply pausing the proceeding could cure the prejudice without the harsh step of exclusion. If a brief adjournment would let the opponent depose a late-disclosed witness or answer a late-disclosed expert, a court may well prefer that lighter touch to preclusion. But a continuance is not a universal solvent. Courts weigh the disruption, the cost, the proximity to trial, and—importantly—the degree to which a continuance would reward the defaulting party's neglect by handing it a do-over at the innocent party's and the court's expense. Where trial is imminent, where the case has already been delayed, or where a continuance would merely shift the burden of the violation onto the blameless side and a congested docket, this factor offers little refuge.

No single factor controls. The court balances all four holistically against the purposes of the disclosure rules—fair notice, an even playing field, and the orderly progress of the case. Design Strategies, Inc. v. Davis, 469 F.3d 284 (2d Cir. 2006), shows the framework in action. There the Second Circuit affirmed the preclusion of evidence and a damages theory that the plaintiff had failed to disclose in discovery, applying the Rule 37(c)(1) factors and concluding that the district court acted well within its discretion in excluding what should have been turned over earlier. Design Strategies is a frequently cited reminder of the rule's core lesson: a party cannot hold back evidence or a theory during discovery and then unveil it at trial; the factors will usually justify keeping the withheld material out.

A national rule: how other circuits frame the analysis

Because Rule 37(c)(1) is a federal rule applied in every district, a litigator outside the Second Circuit—or one whose case may travel on appeal to a different circuit—needs to know that the multi-factor gloss varies in its packaging, though not much in its substance. Two formulations are especially influential.

The Fourth Circuit's test, articulated in Southern States Rack & Fixture, Inc. v. Sherwin-Williams Co., 318 F.3d 592 (4th Cir. 2003), guides the harmlessness and substantial-justification inquiry through five factors: (1) the surprise to the party against whom the evidence would be offered; (2) the ability of that party to cure the surprise; (3) the extent to which allowing the evidence would disrupt the trial; (4) the importance of the evidence; and (5) the nondisclosing party's explanation for its failure to disclose. Southern States is notable for two reasons. First, it folds the "automatic" exclusion analysis into a structured, factor-driven test rather than treating exclusion as a reflex. Second, it makes explicit that a district court need not find bad faith to exclude: the rule does not require willfulness, because its purpose is to protect the opponent from surprise, not merely to punish the discloser. That is a point of real practical importance—a perfectly innocent, good-faith failure can still draw preclusion if it caused incurable surprise.

The Tenth Circuit's test, from Woodworker's Supply, Inc. v. Principal Mutual Life Insurance Co., 170 F.3d 985 (10th Cir. 1999), uses four factors that overlap heavily with the others: (1) the prejudice or surprise to the party against whom the testimony is offered; (2) the ability of that party to cure the prejudice; (3) the extent to which introducing the testimony would disrupt the trial; and (4) the moving (nondisclosing) party's bad faith or willfulness. Woodworker's Supply expressly confirms that the determination of whether a violation is substantially justified or harmless is "entrusted to the broad discretion of the district court."

Lay the three frameworks side by side and the family resemblance is unmistakable. Every version asks, in some order, about the explanation for the failure, the importance of the evidence, the prejudice or surprise to the opponent, and whether that prejudice can be cured (whether by continuance, as the Second Circuit frames it, or by other means). The Fourth Circuit adds an explicit "disruption of trial" factor that the Second Circuit largely subsumes within prejudice and the continuance analysis. The Tenth Circuit foregrounds "bad faith or willfulness," which the Second Circuit treats as relevant to the severity of the appropriate sanction rather than as a freestanding prerequisite. The upshot for a practitioner: master the Patterson four-factor analysis and you have, in substance, the analytical toolkit for any federal court, with only modest local adjustments to vocabulary and emphasis.

A special and common application: experts

Expert disclosures under Rule 26(a)(2) are among the most frequent—and most consequential—settings for Rule 37(c)(1) preclusion, and they deserve their own treatment because the stakes are uniquely high. The Rules demand detailed, timely expert disclosures. For a retained expert (or one whose duties regularly involve giving expert testimony), Rule 26(a)(2)(B) requires a written report containing a complete statement of all opinions to be expressed and the basis and reasons for them; the facts or data considered; any exhibits to be used; the witness's qualifications and publications from the prior ten years; a list of other cases in which the witness testified as an expert in the prior four years; and a statement of the compensation. When a party serves an expert report late, omits opinions from the report and then tries to elicit them at trial, or designates an expert after the deadline, Rule 37(c)(1) is the natural response—and courts regularly preclude the undisclosed or late-disclosed expert testimony.

The reason expert preclusion is so common and so devastating is that expert testimony is frequently indispensable—on causation, on damages, on technical or scientific matters a lay jury cannot assess unaided. Precluding a party's expert can therefore be tantamount to dismissing the very claim that requires expert proof. A plaintiff in a products case who loses its causation expert often loses the case; a trademark plaintiff who loses its consumer-survey expert may lose its best proof of likely confusion. Because the consequence runs so deep, courts apply the Patterson factors with particular care in the expert context, weighing the explanation for the delay, the centrality of the testimony, the prejudice of confronting new opinions late, and whether a continuance could allow a responsive expert and a fresh deposition.

There is an underappreciated interaction here that sophisticated litigators exploit. A disclosure failure under Rule 37(c)(1) and a reliability challenge under Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), and Federal Rule of Evidence 702 are two distinct routes to the same destination—keeping an opponent's expert away from the jury—and a well-prepared motion often pursues both in the alternative. The 37(c)(1) ground argues that the opinion was never properly disclosed; the Daubert ground argues that, disclosed or not, the methodology is unreliable. In trademark practice, both routes converge on the consumer-survey expert, whose work is a perennial target. We explore the reliability attack in detail in Daubert challenges to consumer survey experts in trademark litigation: a practical guide and the underlying methodology in consumer survey expert methodology in trademark cases: a practical guide. The strategic point is simple: if the survey expert's report was served late or omitted the opinions now offered, you may not even need to reach Daubert—Rule 37(c)(1) can exclude the testimony on disclosure grounds alone, and on a far more mechanical showing. The message to practitioners is unambiguous: expert deadlines and report-completeness requirements are not technicalities. A party that treats them casually risks losing its expert—and with the expert, perhaps the case.

Can you rely at trial on something you withheld in discovery?

A recurring and practically vital question—one that arises constantly when one party asks in discovery for the identity of projects, contracts, customers, or documents, and the other party declines to provide them—is whether the withholding party may nonetheless build its case on that very thing at trial. The general answer is no, and the path runs straight through Rule 26(e) and Rule 37(c)(1).

Consider the archetypal scenario. Party A serves an interrogatory asking Party B to identify all projects, contracts, or customers of a certain kind. Party B answers incompletely, omitting a particular item—or objects and refuses to identify it. Later, at trial or on summary judgment, Party B tries to make that very item the centerpiece of its case. Two duties were breached. First, if the item existed and was responsive when the interrogatory was answered, the answer was incomplete. Second, even if Party B learned more afterward, Rule 26(e) imposed a continuing duty to supplement the incomplete answer in a timely fashion. Having failed to disclose the item when required, Party B faces Rule 37(c)(1) preclusion: it generally "is not allowed to use" the undisclosed item to supply evidence at trial or on a motion, unless it can carry its burden of showing substantial justification or harmlessness.

The logic is both doctrinal and equitable. Doctrinally, the item is "information . . . as required by Rule 26(a) or (e)" that was not provided, so the preclusion text applies on its face. Equitably, a party should not be allowed to shield information from its opponent during the very period when the opponent could investigate and respond, and then spring it once that window has slammed shut. Allowing a party to rely on what it refused to identify would reward precisely the gamesmanship the disclosure rules exist to abolish, and would leave the opponent unable to depose witnesses about the item, request related documents, or develop rebuttal. Courts routinely refuse to let a party have it both ways—to treat information as too peripheral to disclose and then too central to omit.

There are, of course, limits and nuances. If the omission was substantially justified—say, the project genuinely did not exist or was not the party's at the relevant time, or a legitimate, sustained, and properly preserved objection excused the answer—or if the opponent learned of the item through other discovery such that the failure was harmless, preclusion may not follow. And the Patterson factors still govern the court's ultimate discretion. But the default expectation is clear, and it should shape behavior: a party that withholds or refuses to identify something in discovery proceeds at its peril if it later wants to use that thing, and the burden will be on it to justify or excuse the nondisclosure. The safest course, always, is to disclose and supplement fully and on time. The riskiest is to bank on later using what you kept hidden.

How to invoke the rule—and how to defend against it

The procedural mechanics of Rule 37(c)(1) are distinctive and frequently botched, so they merit a clear, sequential explanation.

The single most important procedural point bears repeating because so many lawyers get it wrong: no prior motion to compel and no prior court order is required to obtain preclusion under Rule 37(c)(1). This is what sets it apart from Rule 37(b), which authorizes sanctions for violating a discovery order and therefore presupposes an order in the first place. Because 37(c)(1)'s preclusion is self-executing, the moving party need only show that disclosure was required under Rule 26(a) or (e) and did not occur; it need not have first asked the court to compel the disclosure that was withheld. This is a real strategic advantage for the party seeking preclusion—and a genuine trap for the party who assumes that, absent a prior order, its nondisclosure carries no consequence.

The usual vehicle for raising preclusion is a motion in limine filed before trial, asking the court to bar the opponent from using the undisclosed evidence or witness. A motion in limine to exclude evidence on Rule 37(c)(1) grounds is a staple of pretrial practice precisely because it resolves the issue cleanly, before a jury has heard anything it might struggle to un-hear. Preclusion can also be raised by a timely objection at trial when the undisclosed material is offered, or in the summary judgment context by arguing that the opponent cannot rely on undisclosed evidence to create or defeat a genuine dispute of material fact. Timing and diligence cut both ways: a party that knew of a disclosure deficiency and sat on it, or that waited until trial to complain about something it could have raised months earlier, may face arguments that it forfeited or waived the objection, or that its own delay undercuts any claim of prejudice. The prudent course is to identify disclosure failures promptly and tee them up in an orderly fashion before trial—not to hoard the objection as its own form of ambush.

A further procedural distinction concerns the additional sanctions in subparts (A) through (C). While the core preclusion remedy is self-executing, the supplementary sanctions—ordering payment of the expenses (including attorney's fees) caused by the failure, informing the jury of the party's failure, and imposing "other appropriate sanctions"—are available only "on motion and after giving an opportunity to be heard." A party seeking those additional remedies must therefore move for them and afford the opponent a chance to respond. Due process attaches to the more punitive measures, and a court that imposes, say, a costs award without notice and an opportunity to be heard invites reversal.

For the party defending against preclusion, the playbook follows the escape hatches and the factors in a predictable order. Establish substantial justification if a genuine reason truly exists (and resist the temptation to manufacture one—a transparently invented justification damages credibility on every other point). Establish harmlessness by showing the opponent had the substance of the information through other channels or has ample time to respond. Then argue the Patterson factors affirmatively: offer a credible explanation, emphasize the importance of the evidence and the unfairness of exclusion, minimize the prejudice with specifics, and propose a continuance or some lesser sanction as the proportionate cure. Above all, act fast. A party that discovers its own disclosure gap should supplement immediately and proactively, because a prompt, voluntary cure dramatically improves its position on every factor—it bolsters the explanation, shrinks the prejudice, and demonstrates good faith—while continued silence compounds the violation and hardens the court against leniency.

The broader sanctions landscape

Preclusion is the headline remedy, but Rule 37(c)(1) sits inside a larger ecosystem of discovery sanctions, and the additional measures it authorizes can be as significant as exclusion itself. Through subpart (C), the rule incorporates the sanctions listed in Rule 37(b)(2)(A)(i)–(vi), which run the gamut from the moderate to the case-ending: directing that designated facts be taken as established for purposes of the action; prohibiting the disobedient party from supporting or opposing designated claims or defenses, or from introducing designated matters in evidence; striking pleadings in whole or in part; staying further proceedings until the obligation is met; dismissing the action or proceeding in whole or in part; and rendering a default judgment against the disobedient party. The court may also order the offending party to pay the reasonable expenses, including attorney's fees, caused by the failure. And it may inform the jury of the failure—a measure that can be quietly devastating, because it invites the factfinder to draw an adverse inference from a party's discovery conduct, turning a procedural lapse into a credibility wound.

Courts possess this authority because discovery compliance is essential to the system's basic functioning, and that authority is long settled. Update Art, Inc. v. Modiin Publishing, Ltd., 843 F.2d 67 (2d Cir. 1988), recognized the courts' power to impose serious sanctions for discovery violations and articulated the purposes such sanctions serve: ensuring that a party will not benefit from its own failure to comply, securing future compliance with the discovery rules, and deterring others who might be tempted to flout their obligations. Those three purposes—remediation, compliance, and deterrence—animate the entire sanctions regime and explain why courts treat disclosure defaults as something more than paperwork lapses.

The gravity with which courts can regard systematic discovery abuse is illustrated, in the broader national landscape, by decisions like Wachtel v. Health Net, Inc., 239 F.R.D. 81 (D.N.J. 2006), an opinion documenting extensive, pattern-of-conduct discovery misconduct and imposing substantial sanctions. Wachtel arises outside the Second Circuit and addresses abuse far beyond a single nondisclosure, but it underscores a theme that holds everywhere: courts will not tolerate the manipulation of discovery, and the consequences for serious or willful violations escalate accordingly. For the comprehensive treatment of Rule 37's sanctions framework, the standard reference remains Wright & Miller, Federal Practice and Procedure §§ 2289–2299. And because spoliation—the loss or destruction of evidence—travels a parallel but distinct track under Rule 37(e) and the courts' inherent power, it is worth understanding how the regimes differ; we take that up in a companion piece on discovery sanctions under Rule 37 and the related doctrine of spoliation and adverse-inference sanctions.

A worked hypothetical

Consider an invented but realistic scenario that draws the threads together. (It is a hypothetical, offered only to illustrate the analysis.)

A software developer, "NorthBridge," sues a former client, "Harbor Co.," for breach of a development contract, claiming Harbor never paid for a custom platform NorthBridge built. Harbor counterclaims, asserting that NorthBridge's work was defective and that Harbor lost a major customer—"a significant enterprise account"—as a result. During discovery, NorthBridge serves an interrogatory asking Harbor to identify every customer or project Harbor claims to have lost because of NorthBridge's work. Harbor answers vaguely, gesturing at "lost business" in general, and never identifies the specific enterprise account. NorthBridge, taking the answer at face value, does no discovery into any particular lost customer. Discovery closes.

At trial, Harbor seeks to introduce testimony and documents about the specific enterprise account it claims to have lost, quantifying millions in lost revenue tied to that single customer. NorthBridge objects and moves to preclude under Rule 37(c)(1).

Run the analysis. The threshold question is whether disclosure was required and failed. Harbor was asked in an interrogatory to identify lost customers and projects; the specific enterprise account was responsive; Harbor omitted it and never supplemented under Rule 26(e). That is a violation of the disclosure duties the rule enforces. NorthBridge need not have moved to compel earlier, because 37(c)(1) preclusion is self-executing.

The burden now shifts to Harbor to show substantial justification or harmlessness. Substantial justification is weak: Harbor plainly knew the identity of the account it claims to have lost and had no legitimate reason to withhold it; "we described it generally" does not excuse the specific omission of plainly responsive information. Harmlessness is also weak: by hiding the specific account until trial, Harbor deprived NorthBridge of any opportunity to investigate the real reasons for the loss, to depose the customer's representatives, or to develop evidence that the loss had nothing to do with NorthBridge's work (perhaps the customer left over price, or went bankrupt, or was poached by a competitor). The prejudice is acute and, with discovery long closed and trial underway, largely incurable short of derailing the proceeding.

Now the Patterson factors. The explanation is poor—neglect or strategy, not justification. The importance of the evidence is high, but that cuts both ways: the more central the lost account is to Harbor's damages, the more crucial it was to disclose it and the greater the sandbagging. The prejudice to NorthBridge is severe, because it built its entire trial strategy around an interrogatory answer that concealed the key fact. The possibility of a continuance offers little help: trial is underway, and reopening discovery would reward Harbor's withholding while burdening the court and the innocent party. On these facts, a court would be well within its discretion to preclude Harbor from offering evidence about the undisclosed enterprise account—potentially gutting the centerpiece of its damages counterclaim. Harbor could rely on what it refused to identify only if it could justify or excuse the nondisclosure, and here it cannot.

Change the facts and the result flips. Suppose Harbor had identified the account in a timely supplemental interrogatory answer two months before discovery closed; or suppose NorthBridge had learned the account's identity through documents Harbor produced and had in fact deposed the customer's representatives about the lost relationship. Now the failure might be harmless, the prejudice minimal, and preclusion inappropriate—NorthBridge had what it needed, whatever the formal interrogatory said. The lesson is the one that runs through the entire rule: timely, complete disclosure preserves your evidence; concealment forfeits it.

Practical takeaways

For the party seeking preclusion, the strategy is disciplined and front-loaded. Identify the disclosure deficiency precisely—tie it to a specific Rule 26(a) duty (initial, expert, or pretrial) or to an incomplete written response that triggered the Rule 26(e) supplementation obligation. Remember that you need no prior motion to compel; the sanction is self-executing, and you should say so expressly when an opponent argues otherwise. Raise the issue promptly, ordinarily by a motion in limine before trial, and document the prejudice with specifics—what you would have done with timely disclosure (which depositions, which document requests, which rebuttal expert) and exactly what is now foreclosed. Argue the Patterson factors affirmatively rather than reactively, and frame the importance of the evidence as a sword, not just a shield: the more central it is, the worse the sandbagging. Where appropriate, ask not only for preclusion but for the additional sanctions the rule authorizes—fees, an adverse-inference instruction, or more—by motion and with notice. And do not sit on a known deficiency, because delay invites waiver and quietly drains your prejudice argument of force.

For the party defending against preclusion, speed and candor are everything. The moment you discover a gap in your own disclosures, supplement immediately and completely; a proactive cure improves your standing on every factor at once, while continued silence deepens the violation and forfeits the court's goodwill. Be ready to carry your burden on the escape hatches: marshal a genuine substantial-justification argument if one truly exists, and build the harmlessness case by showing the opponent already had the substance of the information or still has ample time to respond. On the Patterson factors, offer a credible explanation (a real one—courts can smell a manufactured excuse), stress the importance of the evidence and the harshness of exclusion, minimize the prejudice with concrete detail, and propose a continuance or a lesser sanction as a proportionate alternative to the "drastic remedy" of preclusion. Never assume that the absence of a prior court order means your nondisclosure is consequence-free—because under Rule 37(c)(1), it emphatically is not.

For both sides, the deepest lesson is cultural rather than tactical. The disclosure rules are engineered to make litigation a contest of merits, not of surprises, and Rule 37(c)(1) gives that design teeth. A litigant who internalizes the duty to disclose fully and to supplement diligently—who treats discovery as a process of orderly exchange rather than a game of concealment—rarely finds itself on the wrong end of a preclusion motion. A litigant who treats discovery as hide-and-seek eventually discovers that the rule's bargain is real and unsentimental: what you fail to disclose, you generally cannot use. In a system that prizes fair notice and a level playing field, that is not a technicality. It is the whole point.

Frequently asked questions

Do I have to win a motion to compel before I can preclude undisclosed evidence? No. This is the most common misconception about the rule. Rule 37(c)(1) preclusion is self-executing and requires no prior motion to compel and no prior court order—that is what distinguishes it from sanctions under Rule 37(b), which presuppose the violation of an order. You need only show that disclosure was required under Rule 26(a) or (e) and did not happen. The burden then shifts to your opponent to prove substantial justification or harmlessness.

Who bears the burden of proof on a preclusion motion? Once the moving party shows that disclosure was required and did not occur, the non-disclosing party bears the burden of establishing that its failure was substantially justified or harmless. See Yeti by Molly, Ltd. v. Deckers Outdoor Corp., 259 F.3d 1101, 1107 (9th Cir. 2001). This allocation is the opposite of the usual intuition and is one of the rule's most powerful features for the party seeking exclusion.

Is preclusion automatic, or does the judge have discretion? Both, properly understood. The availability of preclusion follows automatically from a violation that is neither justified nor harmless, but the choice whether to impose preclusion—as opposed to a continuance, a fee award, or another lesser sanction—rests in the district court's sound discretion. In the Second Circuit, that discretion is structured by the four Patterson v. Balsamico factors. Courts describe preclusion as a "harsh" or "drastic" remedy, especially when it would effectively decide the case.

What is the difference between "substantially justified" and "harmless"? "Substantially justified" looks at why the disclosure failed—whether there was a reasonable basis in law and fact for the lapse, in the sense of Pierce v. Underwood, 487 U.S. 552, 565 (1988). "Harmless" looks at effect—whether the failure actually prejudiced the opponent. They are independent: either one alone defeats preclusion. A good reason can excuse a lapse that caused some prejudice; an absence of prejudice can excuse a lapse that had no good reason.

Can the rule exclude evidence at summary judgment, or only at trial? At both. The text bars use of the undisclosed information or witness "on a motion, at a hearing, or at a trial." Because the summary judgment record is so frequently dispositive, preclusion can decide a case long before any trial—knocking out, for example, an undisclosed damages computation or a document the disclosing party never produced.

Does the rule require bad faith or willfulness? No. As the Fourth Circuit emphasized in Southern States Rack & Fixture, Inc. v. Sherwin-Williams Co., 318 F.3d 592 (4th Cir. 2003), the purpose of the rule is to protect the opponent from surprise, not solely to punish the discloser. A good-faith but unjustified failure that caused incurable surprise can still draw preclusion. Bad faith or willfulness aggravates the violation and pushes toward the most severe sanctions, but it is not a prerequisite to exclusion.

How do I actually raise preclusion? Most often through a motion in limine before trial, asking the court to bar the opponent from using the undisclosed evidence. You can also object at trial when the material is offered, or argue at summary judgment that your opponent cannot rely on undisclosed evidence to create or defeat a genuine dispute. Raise it promptly—delay can forfeit the objection and undercut your prejudice argument.

My opponent disclosed an expert report late. Is the testimony automatically out? Not automatically, but late or incomplete expert disclosures under Rule 26(a)(2) are among the most common grounds for preclusion, and the consequence can be case-ending where the claim requires expert proof. The court will weigh the Patterson factors. Consider pairing the disclosure objection with a reliability challenge under Daubert and Rule 702; the two grounds are independent and often briefed together.

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This article is provided for general informational purposes and does not constitute legal advice. The application of Rule 37(c)(1) is intensely fact-specific and committed to the discretion of the trial court; consult qualified litigation counsel about any particular matter.