A procedural mechanism by which a group of similarly-situated plaintiffs (or defendants) may prosecute (or defend) a lawsuit as a class, instead of as individuals. Federal Rule of Civil Procedure (FRCP) 23 governs class actions in federal courts. States often have their own class action procedures, which may be similar to FRCP 23.

A federal lawsuit styled as a class action does not actually become a class action unless the court enters an order certifying that the case satisfies the criteria set out in FRCP 23(a) and (b). In addition, federal courts often require a showing that any certified class is sufficiently definite, in other words, that the members are ascertainable. For more information about certifying a class action in federal court, see Practice Note, Class Actions: Certification.

A final judgment on the merits in a class action generally is binding on all members of the class, provided due process has been met. Members of classes certified under FRCP 23(b)(3), however, are given the opportunity to opt out of the class action.

Unlike individual litigation, a federal court must approve a class action settlement to ensure that the proposed settlement is fair, reasonable, and adequate (FRCP 23(e)). For more information about settling class actions in federal court, see Practice Note, Settling Class Actions: Process and Procedure.