In law, a class action or a representative action is a form of lawsuit where a large group of people collectively bring a claim to court. This form of collective lawsuit originated in the United States and is still predominately a US phenomenon, at least the US variant of it. However, in several European countries with civil law (as opposed to the Anglo-Saxon common law principle, which is used by US courts), changes have in recent years been made that allow consumer organisations to bring claims on behalf of large groups of consumers.

Federal class actions

In the United States federal courts, class actions are governed by Federal Rules of Civil Procedure Rule 23 and 28 U.S.C.A. § 1332 (d).

Class action lawsuits may be brought in federal court if the claim arises under federal law, or if the claim falls under 28 USCA § 1332 (d). Under § 1332 (d) (2) the federal district courts have original jurisdiction over any civil action where the amount in controversy exceeds $5,000,000 and either 1. any member of a class of plaintiffs is a citizen of a State different from any defendant; 2. any member of a class of plaintiffs is a foreign state or a citizen or subject of a foreign state and any defendant is a citizen of a State; or 3. any member of a class of plaintiffs is a citizen of a State and any defendant is a foreign state or a citizen or subject of a foreign state. Nationwide plaintiff classes are possible, but such suits must have a commonality of issues across state lines. This may be difficult if the civil law in the various states have significant differences. Large class actions brought in federal court frequently are consolidated for pre-trial purposes through the device of multidistrict litigation (MDL).Fact: date=March 2008 It is also possible to bring class action lawsuits under state law, and in some cases the court may extend its jurisdiction to all the members of the class, including out of state (or even internationally) as the key element is the jurisdiction that the court has over the defendant.

Typically, federal courts are thought to be more favorable for defendants, and state courts more favorable for plaintiffs. Many class action cases are filed initially in state court. The defendant will frequently try to remove the case to federal court. The Class Action Fairness Act of 2005 increases defendants' ability to remove state cases to federal court by giving federal courts original jurisdiction for all class actions with damages exceeding $5,000,000, exclusive of interest and costs. It should be noted, however, that the Class Action Fairness Act contains carve-outs for, 'inter alia', shareholder class action lawsuits covered by the PSLRA and those concerning internal corporate governance issues (the latter typically being brought as shareholder derivative actions in the state courts of Delaware, the state of incorporation of most large corporations).