The rules of civil procedure or the code of civil procedure in state and federal courts usually permit a party to a civil lawsuit (a litigant) to amend its pleadings.

A plaintiff can amend the complaint or petition to assert new claims or causes of action before or during trial, subject to certain limitations—and the defendant can amend the answer to the lawsuit to assert new defenses to the plaintiff's claims before or during trial, subject to certain limitations.

In Missouri, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Missouri Rules of Civil Procedure, specifically Rule 55.33, a party may amend its pleading once as a matter of course before a responsive pleading is served, or if the pleading is one to which no responsive pleading is permitted and the action has not been placed upon the trial docket, the party may so amend it within 30 days after it is served. Otherwise, a party may amend its pleading only by leave of the court or by written consent of the adverse party, and leave shall be freely given when justice so requires. At the federal level, Rule 15 of the Federal Rules of Civil Procedure provides similar allowances for amending pleadings. It states that a party may amend its pleading once as a matter of course within 21 days after serving it, or if the pleading is one that requires a response, within 21 days after service of a responsive pleading or 21 days after service of a motion under Rule 12(b), (e), or (f), whichever is earlier. In both Missouri state and federal courts, the decision to allow amendments is typically subject to the court's discretion and may consider factors such as the timing of the amendment, the reasons for the amendment, and potential prejudice to the opposing party.