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. A lawsuit that allows a large number of people with a common interest in a matter to sue or be sued as a group.

The class action suit began in the Equity courts of seventeenth-century England as a bill of peace. English courts would allow a bill of peace to be heard if the number of litigants was so large that joining their claims in a lawsuit was not possible or practical.

Class action lawsuits may offer a number of advantages because they aggregate a large number of individualized claims into one representational lawsuit. This type of lawsuit has a legitimate social purpose. A lawyer who prosecutes a class action can be viewed as a "private attorney general" who aggressively enforces various regulatory laws or who alerts the public to Fraud, health, and safety problems. A class action is an effective means for holding defendants accountable for widespread harm that would otherwise go unchecked. There is public value in allowing this type of class action to go forward, even if the amount payable to each member of the class is small. a class action avoids the situation where different court rulings could create "incompatible standards" of conduct for the defendant to follow.

There are several criticisms of class action lawsuits. Class members often receive little or no benefit from class actions. Class action lawsuits have become a controversial topic in the 1990s. Critics charge that class actions have been used by defendants in mass TORT cases, such as asbestos litigation, to frustrate the large and legitimate claims of individual victims. Critics of class actions remain unconvinced about the social and legal value of group lawsuits. In small claims class actions, critics question the value of supporting litigation in which individual class members have very small stakes. Critics also dispute the value of the private attorney general role. Most class action attorneys, they contend, are seeking lucrative financial awards rather than social justice. As to large claim class actions, critics believe that the victims may not be fairly served.

In American law a lawsuit is a civil action brought before a court in which the party commencing the action, the plaintiff, seeks a legal or equitable remedy. A lawsuit may involve dispute resolution of private law issues between individuals, business entities or non-profit organizations. A lawsuit may also enable the government to be treated as if it were a private party in a civil case.

Rules of criminal or civil procedure govern the conduct of a lawsuit in the common law adversarial system of dispute resolution. Procedural rules are additionally constrained/informed by separate statutory laws, case law, and constitutional provisions. The rules are very important for litigants to know, however, because they dictate the timing and progression of the lawsuit

Though the majority of lawsuits are settled and never even get to trial, they can expand into a very complicated process. This is particularly true in federal systems, where a federal court may be applying state or vice versa. Lawsuits become additionally complicated as more parties become involved. Courts typically have some power to separate out claims and parties into separate suits if it is more efficient to do so, such as if there is not a sufficient overlap of factual issues between the various claims.

In the United States federal courts, class actions are governed by Federal Rules of Civil Procedure Rule 23. 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). 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).

It is also possible to bring class action lawsuits under state law. 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 procedure for filing a class action is to file suit with one or several named plaintiffs on behalf of a proposed class. After the complaint is filed, the plaintiff must file a motion to have the class certified.