class action, in law, a device that permits one or more persons to sue or be sued as representative of a large group of people interested in the matter at issue. The court in whose jurisdiction a suit is brought typically has wide discretion in determining that a class will be so represented. Certain requirements must be met, e.g., the class must be so large or dispersed that actual joinder of all individuals would be impractical; there must be questions of law and fact common to all members, and these must outweigh any individual questions; and the named parties must adequately represent the interests of their class. Certain forms of notice to members of the class, e.g., by newspaper or broadcast publication or by mail, are also required. In most types of suit, all members of the class are bound by the decision, unless a member of the class opted out of the action at the beginning of the lawsuit. An absentee member may be able to contest the outcome on the basis that due process of law was not adhered to.

In the United States, federal and most state courts allow class action suits. Such suits have figured prominently in civil-rights litigation and in other cases brought to further social and economic reform. In recent decades they have been employed notably by groups of consumers and others seeking to affix liability for harm caused by various products, especially through manufacturers' negligence. Major litigation against the producers of the Dalkon shield (an intrauterine device; see birth control), of Agent Orange (a herbicide used as a defoliant in the Vietnam War), and of asbestos insulation has involved class action suits.

Since the 1980s such suits have been under attack, along with negligence litigation in general, with opponents, mainly conservatives and business interests, arguing that many lawsuits are frivolous and that awards are out of proportion to the offense in some juridictions. A study published in 2004 that reviewed several hundred state and federal class action lawsuits from 1993 to 2002 found that, adjusted for inflation, the average annual award in such suits varied but did not progressively increase, while the median award was relatively constant. At the same time, however, federal court data showed that the number of class action lawsuits doubled from 1997 to 2002.