The defendant in a lawsuit must be served (delivered) a summons or citation (an order from the court to appear and answer the lawsuit) and a copy of the lawsuit (the complaint) (1) to provide due process (fairness) by giving the defendant notice of the lawsuit and the opportunity to appear in court and defend the lawsuit; and (2) to give the court personal jurisdiction (authority) over the defendant.

In Missouri, as in all states, the due process clause of the Constitution requires that a defendant in a lawsuit be properly notified of the legal action against them and be given an opportunity to appear in court and defend themselves. This is accomplished through the service of a summons or citation, along with a copy of the complaint (the document outlining the plaintiff's allegations). The summons informs the defendant that they are being sued and provides instructions on how to respond to the lawsuit. Missouri's rules for service of process are outlined in Missouri Rules of Civil Procedure. Generally, service can be made personally, by leaving the documents at the defendant's dwelling with someone of suitable age and discretion, or by mail, among other methods. Proper service ensures that the court has personal jurisdiction over the defendant, meaning the court has the authority to make decisions affecting the defendant's rights. If a defendant is not properly served, they may challenge the court's jurisdiction, and the case may be dismissed or the service may have to be completed again.