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 Illinois, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to the Illinois 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 either before or during the trial. However, there are limitations to these amendments. The court may deny an amendment if it finds that it would cause prejudice to the other party, if it is sought in bad faith, or if it would be futile. Additionally, after a certain point in the litigation process, such as after a case management conference or discovery deadlines, the court's permission is typically required to make amendments, and the court will consider factors such as the timeliness of the request and the reasons for the delay. At the federal level, the Federal Rules of Civil Procedure similarly permit amendments to pleadings, with Rule 15 outlining the general policy favoring the allowance of amendments to ensure that cases are decided on their merits rather than on technicalities. However, as with state rules, amendments are subject to the court's discretion and may be denied if they would result in undue delay, prejudice, or are sought in bad faith.