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 Alaska, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Alaska Rules of Civil Procedure. Rule 15 provides that 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. Beyond this, parties must seek the opposing party's written consent or the court's permission to amend, which the court should freely give when justice so requires. This is consistent with the principle of allowing parties to fully present their cases, as long as the amendment does not cause undue prejudice to the other party. At the federal level, the Federal Rules of Civil Procedure Rule 15 operates similarly, allowing for amendments to pleadings with the court's permission, which should be freely given when justice requires, and setting forth the timing and conditions under which amendments may be made. Both state and federal rules aim to ensure that cases are decided on their merits rather than on technicalities, while also considering the potential prejudice to the opposing party.