In the law, a cause of action is a recognized kind of legal claim that a plaintiff pleads or alleges in a complaint to start a lawsuit. Examples are: breach of contract; torts such as injury , fraud, slander; suits at equity. "Cause of action" encompasses both the legal theory of what legal wrong the plaintiff claims to have suffered, and the remedy, which is what a court is allowed to order the defendant to do to compensate the plaintiff for that wrong.

The points a plaintiff must prove to win a given type of case are called the "elements" of that cause of action. For the cause of action of negligence, for example, the elements are (existence of a) duty, breach (of that duty), causation (by that breach), and damages (incurred by the plaintiff). If a complaint does not allege facts to support every element of the cause of action it describes, the court will dismiss the complaint for failure to state a claim, for which relief can be granted.

The respondent to a cause of action may plead denials or affirmative defenses. Most defenses must be raised in the pleadings or by motion or are waived at trial. A few defenses, in particular a court's lack of subject matter jurisdiction, need not be pleaded and may be raised at any time.