Federal Rule of Civil Procedure 37(c)(1) is the enforcement engine that retired the era of "trial by ambush." A party that fails to disclose information or a witness required by Rule 26(a), or to supplement under Rule 26(e), generally may not use that information or witness to supply evidence on a motion, at a hearing, or at trial—unless the failure was substantially justified or harmless. This guide explains, in plain language for litigators, business owners, judges, and policymakers, how the rule actually operates: the affirmative disclosure duties it backstops, its self-executing character, the two escape hatches and who must prove them, and the multi-factor tests the circuits use—the Second Circuit's Patterson/Softel factors, the Fourth Circuit's Southern States framework, and the Tenth Circuit's Woodworker's Supply factors. It works through the high-stakes expert context under Rule 26(a)(2), the recurring problem of undisclosed damages computations, the question whether a party may rely at trial on something it withheld in discovery, the procedural mechanics of a preclusion motion, the menu of lesser sanctions, and how 37(c)(1) differs from Rule 37(b), (c)(2), and (e). Two worked hypotheticals, side-by-side checklists for both sides, an FAQ, and curated cross-references round out a research-grade treatment.