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 Arizona, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Arizona Rules of Civil Procedure. Rule 15 of these rules allows a party to amend its pleadings once as a matter of course within a certain time frame—specifically, either before a responsive pleading is served or within 20 days after the pleading is served if no responsive pleading is required. Beyond this, parties may amend their pleadings only with the opposing party's written consent or the court's leave, which the court should freely give when justice so requires. This is to ensure that all claims and defenses can be fully developed and considered. At the federal level, the Federal Rules of Civil Procedure also provide for the amendment of pleadings. Rule 15 of the Federal Rules is similar to Arizona's, allowing one amendment as a matter of course early in the proceedings and otherwise requiring consent or leave of court. Both sets of rules recognize that circumstances may change as litigation progresses, and they aim to balance the need for procedural fairness with the need to resolve disputes on their merits.