In 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. One or more defendants are required to respond to the plaintiff complaint. If the plaintiff is successful, judgment will be given in the plaintiff's favor, and a range of court orders may be issued to enforce a right, award damages, or impose an injunction to prevent an act or compel an act. A declaratory judgment may be issued to prevent future legal disputes.

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, as plaintiff or defendant regarding an injury, or may provide the government with a civil cause of action to enforce certain laws.

The conduct of a lawsuit is called litigation.

Rules of procedure and complications in lawsuits

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 that define the rights of the parties to a lawsuit (see especially due process), though the rules will generally reflect this legal context on their face. The details of procedure will differ from jurisdiction to jurisdiction, and often from court to court within the same jurisdiction. The rules are very important for litigants to know, however, because they dictate the timing and progression of the lawsuit what may be filed and when to get what result. Failure to comply with the procedural rules can result in serious limitations in conducting the trial or even dismissal of the lawsuit.

Though the majority of lawsuits are settled and never even get to trialFact: date=September 2007, they can expand into a very complicated process. This is particularly true in federal systems, where a federal court may be applying state law (e.g., the Erie doctrine in the United States) or vice versa, or one state applying the law of another, and where it additionally may not be clear which level (or location) of court actually has jurisdiction over the claim or personal jurisdiction over the defendant. Domestic courts are also often called upon to apply foreign law, or to act upon foreign defendants, over whom they may not, as a practical matter, even have the ability to enforce a judgment if the defendant's assets are outside their reach.

Lawsuits become additionally complicated as more parties become involved (see joinder). Within a "single" lawsuit, there can be any number of claims and defenses (all based on numerous laws) between any number of plaintiffs or defendants, who each can bring any number of cross-claims and counterclaims against each other, and even bring additional parties into the suit on either side after it progresses. However, 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.