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 Colorado, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Colorado Rules of Civil Procedure (CRCP) Rule 15, 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, amendments require either consent from the opposing party or leave from the court, which should be freely given when justice so requires. However, amendments may be denied if they cause undue delay, prejudice the opposing party, or are futile. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 provides similar provisions for amending pleadings, with the philosophy that cases should be decided on their merits rather than on technicalities. Both state and federal rules recognize that circumstances may change as a case progresses, and they provide flexibility for parties to assert new claims or defenses, while also seeking to prevent abuse of the process and to ensure fairness to all parties involved.