Class certification is the pivotal ruling in aggregate litigation and requires a rigorous analysis that frequently overlaps with the merits.

Prerequisites. Numerosity, such that joinder is impracticable; commonality, meaning questions capable of classwide resolution that will generate common answers; typicality of the representative’s claims; and adequacy of the representative and counsel.

Implicit requirements. An ascertainable or administratively feasible class definition in some circuits, and standing of the named plaintiff.

Type one. Separate actions would risk inconsistent adjudications binding the defendant, or would impair absent members’ interests — the limited fund and inconsistent obligations categories. No opt-out.

Type two. Injunctive or declaratory relief appropriate for the class as a whole. Not available where monetary relief is more than incidental.

Type three. Common questions predominate over individual ones, and a class action is superior to other methods. Predominance is where most commercial class actions are decided, and the availability of a common method of proving damages consistent with the liability theory is central.

Damages models must measure only the damages attributable to the theory of liability certified.

Interlocutory review of certification orders is available by petition within a short period, and is one of the few immediate appeals in civil practice.