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 Wyoming, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Wyoming Rules of Civil Procedure. Rule 15 of these rules allows a party to amend its pleadings, which includes a complaint or petition by the plaintiff and an answer by the defendant. Amendments can be made to assert new claims, causes of action, or defenses. 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 should be freely given when justice so requires. However, the court may deny an amendment if it results in undue delay, prejudice to the opposing party, or is futile. At the federal level, the Federal Rules of Civil Procedure also provide similar provisions under Rule 15, allowing for amendments to pleadings with the same spirit of liberality, subject to the discretion of the federal court to deny amendments for reasons such as undue delay, bad faith, or dilatory motive on the part of the movant.