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 Oregon, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Oregon Rule of Civil Procedure (ORCP) 23, 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 their pleading only by leave of the court or by written consent of the adverse party; 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. Amendments before trial are freely allowed when justice requires, and specific time frames are set for amending pleadings with or without the court's leave. Both state and federal rules aim to ensure that cases are decided on their merits rather than on technicalities, but they also seek to prevent undue delay or prejudice to the opposing party.