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 Louisiana (LA), the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to the Louisiana Code of Civil Procedure, a plaintiff may amend their petition without leave of court within 10 days of service of the original petition, or if the petition has not yet been served, at any time before service. After this period, or once a responsive pleading is served, the plaintiff must seek the court's permission to amend, which the court may grant if it is just and reasonable. Similarly, a defendant may amend their answer within 10 days of serving the original answer, and thereafter with the court's permission. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 allows parties to amend their pleadings once as a matter of course within 21 days after serving it, or if a response is required, within 21 days after service of a responsive pleading or a pretrial motion under Rule 12(b), (e), or (f). Subsequent amendments generally require the opposing party's written consent or the court's leave, which the court should freely give when justice so requires. Both state and federal rules aim to ensure that cases are decided on their merits rather than on procedural technicalities, but they also seek to prevent undue delay or prejudice to the opposing party.