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 New Mexico, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Rule 1-015 NMRA (New Mexico Rules of Civil Procedure for the District Courts), a party may amend its pleading once as a matter of course within 30 days after serving it, or if the pleading is one to which a responsive pleading is required, within 30 days after service of the responsive pleading or pretrial order, whichever is earlier. Beyond this, amendments require either consent from the opposing party or leave from the court, which the court should freely give when justice so requires. There are limitations, however, such as the need to avoid undue delay, prejudice to the opposing party, or futility of the amendment. At the federal level, Rule 15 of the Federal Rules of Civil Procedure provides similar provisions for amending pleadings, with the philosophy that cases should be decided on their merits rather than the technicalities of pleadings. Both state and federal rules recognize that circumstances may change as a case progresses, and the rules are designed to allow parties to present their claims and defenses as fully as possible, subject to the court's discretion to prevent abuse of the process.