Rule 53.01.Commencement of Civil Action.

Rule 53.01 is one sentence, and it settles a foundational question in the plainest terms: a civil action is commenced by filing a petition with the court. Filing, not service on the defendant and not any other formal step, is the act that starts the case. The official comment notes that this corresponds to the federal rule on commencing an action, with the Missouri term "petition" substituted for the federal term "complaint."

That single moment carries weight well beyond the courthouse doorstep. It is generally the point that stops a statute of limitations from running against the claims stated in the petition, and it anchors many other procedural deadlines throughout the rules that are measured from when an action is "commenced." Fixing that moment at filing, instead of at service or at some later point, gives plaintiffs and defendants alike a clear, verifiable date the court's own docket records.

Commencement and service are distinct events under Missouri procedure. Filing the petition opens the case with the court; separate rules elsewhere govern how and when the defendant must be served with process once the case exists. A plaintiff who files a petition has commenced an action even before the defendant has been served or has learned of the suit at all.