The Summons is the prepared documents that inform a defendant that he or she is being sued and asserts the power of the court to hear and determine the case. These forms are of legal process that commands the defendant to appear before the court on a specific day and to answer the complaint made by the plaintiff.

The summons is the document that officially starts a lawsuit. It must be in a form prescribed by the law governing procedure in the court involved, and it must be properly served on, or delivered to, the defendant. If the prescribed formalities are not observed, the court lacks authority to hear the dispute.

The Complaint is any formal legal document that sets out the facts and legal reasons (see: cause of action) that the filing party or parties (the plaintiff(s)) believes are sufficient to support a claim against the party or parties against whom the claim is brought (the defendant(s)) that entitles the plaintiff(s) to a remedy (either money damages or injunctive relief)]). For example, the Federal Rules of Civil Procedure (FRCP) that govern civil litigation in United States courts provide that a civil action is commenced with the filing or service of a pleading called a complaint. Civil court rules in states that have incorporated the Federal Rules of Civil Procedure use the same term for the same pleading.

In the federal district courts, the summons is prepared by the attorney for the plaintiff and given to the clerk of the court where the case will be heard. When the plaintiff's complaint, setting out his claim, is filed with the court, the clerk signs the summons and gives it and a copy of the complaint to a U.S. marshal or to someone else appointed to serve the papers. Once the summons and complaint are served on the defendant, she must respond to them within twenty days or whatever other time the court allows.

Some states follow this same procedure, but other states allow service of the summons and complaint by delivery directly to the defendant. In those states, the lawsuit is considered begun as soon as the defendant receives the papers.