A class action lawsuit is a type of legal proceeding where one or more plaintiffs bring a lawsuit on behalf of a larger group, known as the class. In a typical class action, a plaintiff sues a defendant or a number of defendants on behalf of a group, or class, of absent parties. This differs from a traditional lawsuit, where one party sues another party, and all of the parties are present in court. Class actions are most common where the allegations usually involve at least 40 people who the same defendant has injured in the same way. The class action originated in the United States and is still predominantly an American phenomenon, but Canada, as well as several European countries with civil law, have made changes in recent years to allow consumer organizations to bring claims on behalf of consumers.

Class actions are practical for the plaintiffs as they mean just one set of witnesses, one set of experts, one set of documents, and one set of issues. This efficiency makes it less expensive and easier for one law firm to handle one case than having one or more law firms try multiple cases. Class actions are also practical for the courts because one lawsuit is cheaper for the courts than multiple lawsuits. One lawsuit means just one judge and one courtroom. Class actions result in judgments or settlements that bind all class members, even those unaware of the action in many cases. A class action uniquely can bind someone to its outcome without the person ever receiving notice of the action, contrary to the typical requirements of procedural due process.

To start a class action lawsuit, one or more named plaintiffs represent a large group. The named plaintiffs file the case and are actively involved in legal proceedings. A much larger group of unnamed plaintiffs have shared legal claims with the named plaintiffs. They can choose to be a part of the class and have their claims resolved by it, or they can choose to opt-out and pursue their lawsuits for the wrongs committed against them. Class actions can be brought in state or federal court, and they are allowed only in civil cases where plaintiffs seek monetary remedies.