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 California, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. Under the California Code of Civil Procedure, plaintiffs may amend their complaint or petition to assert new claims or causes of action, and defendants can amend their answers to assert new defenses. These amendments can be made before or during the trial. However, there are limitations to these amendments. For instance, after a certain point in the proceedings, such as after a demurrer is sustained without leave to amend or after a certain number of amendments have already been made, the court's permission may be required to make further amendments. The court will consider factors such as whether the amendment would cause prejudice to the other party, the timing of the amendment, and whether there has been a previous opportunity to amend. At the federal level, the Federal Rules of Civil Procedure similarly allow for amendments to pleadings, with Rule 15 addressing when and how a party may amend a pleading and the considerations for the court in granting or denying such amendments.