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 Alabama, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Alabama 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 within 21 days after serving it, or if the pleading is one to which a responsive pleading is required, 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. Otherwise, a party must seek the court's permission to amend, and the court is guided by the principle of allowing amendments to promote justice and facilitate the effective resolution of disputes. The federal courts operate under similar principles, as outlined in the Federal Rules of Civil Procedure, specifically Rule 15, which also encourages liberality in allowing amendments to pleadings to ensure that cases are decided on their merits rather than on technicalities. However, both state and federal courts may deny amendments if they result in undue delay, prejudice to the opposing party, or are futile.