Federal Rule of Evidence 403 lets a trial judge exclude relevant evidence when its probative value is substantially outweighed by unfair prejudice, confusion, misleading the jury, undue delay, waste of time, or needless cumulativeness. It is the most heavily litigated rule in an American courtroom, and it governs most of the pretrial fights that decide what a jury ever sees. This guide dissects the balance word by word, shows what the Supreme Court's decision in Old Chief v. United States teaches about both pans of the scale, and applies the rule to the disputes that recur in commercial and trademark cases: demonstratives and the new Rule 107, a party's wealth and size, character and other-acts evidence under Rules 404 and 609, and a litigant's history of lawsuits. It covers the motion in limine as the vehicle, abuse-of-discretion review on appeal, and the toolkit of measures short of exclusion, and closes with a worked hypothetical, an FAQ, and cross-references to related UpLaw resources.