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 Washington State, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Washington Civil Rules of Procedure (CR). Specifically, CR 15 allows parties to amend their pleadings, which includes a plaintiff's complaint or petition and a defendant's answer. Amendments may be made to assert new claims, causes of action, or defenses. The rule generally encourages amendments to be granted freely when justice so requires, but there are limitations. For instance, an amendment may not be allowed if it causes undue delay, prejudice to the opposing party, or is futile. Additionally, after a certain point in the litigation process, typically after the responsive pleading or as the trial date approaches, the party must seek the court's permission to amend. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 similarly governs amendments to pleadings, allowing for changes to be made with the court's leave or the opposing party's written consent, and it also emphasizes that leave should be freely given when justice requires.