In a civil action, the document that initiates a lawsuit. The complaint outlines the alleged facts of the case and the basis for which a legal remedy is sought. In a criminal action, a complaint is the preliminary charge filed by the complaining party, usually with the police or a court.

The complaint is the first court paper filed in a lawsuit. It briefly states the plaintiff's view of the crux of the legal dispute and asks the court to resolve the dispute. In some types of cases and in certain states, a complaint is called a petition or a libel.

Items that typically appear in a complaint include:

Caption. The caption is the heading which appears on all court papers. The caption contains the names of the parties to the lawsuit (for example, Susan Roe, Plaintiff, v. Robert Roe, Defendant), the name of the court (for example, District Court for the Eastern District of Pennsylvania), the case number which has been assigned by the court clerk, and the title of the court paper (for example, Complaint for Annulment).

Allegations. An allegation is a statement made in court papers that sets forth a party's belief as to what the facts are in a given case. Referring to statements made in court papers as allegations serves as a reminder that they may or may not be true. Thus, when a party has alleged something, she has made charges which remain to be proven.

Prayer. The prayer is the part of a complaint which requests the court to grant some specific judicial relief (for example, a divorce, possession of the family home, child support or custody).

The allegation made to a proper officer that some person, whether known or unknown, has been guilty of a designated offence, with an offer to prove the fact and a request that the offender may be punished.

To have a legal effect the complaint must be supported by such evidence as shows that an offence has been committed, and renders it certain or probable that it was committed by the person named or described in the complaint.