A class action is a type of lawsuit in which one or a few plaintiffs seek to represent hundreds or thousands of plaintiffs with the same or similar claims against the same defendants. Rules of procedure in state and federal courts permit class actions in limited circumstances for the purpose of resolving similar disputes more efficiently than if each plaintiff filed a separate lawsuit.

In Illinois, class action lawsuits are governed by the Illinois Code of Civil Procedure, specifically under 735 ILCS 5/2-801. This statute outlines the requirements for a class action, which include the necessity for a class that is so numerous that individual joinder of all members is impracticable, common questions of law or fact, typical claims or defenses of the representative parties, and the representative parties will fairly and adequately protect the interests of the class. Additionally, the class action must be an appropriate method for the fair and efficient adjudication of the controversy. At the federal level, class actions that meet certain criteria may be governed by the Federal Rules of Civil Procedure, Rule 23. This rule also requires numerosity, commonality, typicality, and adequacy of representation. Moreover, for a class action to proceed in federal court, it must meet one of the conditions outlined in Rule 23(b), such as the party opposing the class has acted or refused to act on grounds that apply generally to the class. Class actions can be a powerful tool for plaintiffs with similar claims, allowing them to pool resources and litigate their claims more efficiently. However, they must meet strict procedural requirements to be certified as a class by the court.