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 Utah, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Utah Rules of Civil Procedure. Rule 15 of these rules allows a party to amend its pleadings, which includes a plaintiff amending a complaint or a defendant amending an answer. Amendments may be made to assert new claims, causes of action, or defenses. Generally, 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, the party may amend it within 21 days after it is served. Otherwise, a party must seek the opposing party's written consent or the court's leave to amend, which the court should freely give when justice so requires. There are limitations to these amendments, especially when they would cause undue delay, prejudice to the opposing party, or when they are futile. At the federal level, similar provisions are found in the Federal Rules of Civil Procedure, specifically Rule 15, which also encourages courts to grant leave to amend when justice requires and outlines similar timing and restrictions.