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 Montana, as in other states, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to the Montana Rules of Civil Procedure (MRCP), specifically 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 the court should freely give when justice so requires. This is intended to ensure that cases are decided on their merits rather than on technicalities. 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) provide similar provisions under Rule 15, allowing for amendments to pleadings with the court's permission, which should be freely given when justice requires. Both state and federal rules aim to balance the right to amend pleadings with the need to avoid undue delays and prejudice, ensuring a fair trial process.