In the common law, a complaint is a formal legal document that sets out the basic facts and legal reasons (see: cause of action) that the filing party (the plaintiffs) believes are sufficient to support a claim against another person, persons, entity or entities (the defendants) that entitles the plaintiff(s) to a remedy (either money damages or injunctive relief). See also petition.

In criminal cases, it is a criminal complaint (felony complaint) and the plaintiff is the State (also sometimes called the People) or Crown (in commonwealth countries) as the source of the prosecutor's authority emanates from the police power of the state with the defendant being the accused who faces criminal sanctions.