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 Kansas, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Kansas Statutes Annotated (K.S.A.) and the Kansas Rules of Civil Procedure, 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 the court should freely give when justice so requires. However, there are limitations, especially when amendments seek to add new claims or defenses that would unduly delay the proceedings or prejudice the opposing party. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 similarly allows for amendments to pleadings, with the philosophy of favoring decisions on the merits rather than on procedural technicalities. Amendments before trial are generally permitted if they do not cause undue delay or prejudice to the opposing party. The specific timing and conditions for amendments are detailed in FRCP Rule 15.