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 West Virginia, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Rule 15 of the West Virginia Rules of Civil Procedure, a party may amend its pleading once as a matter of course within 20 days after it is served, or if the pleading is one to which a responsive pleading is required, within 20 days after service of the responsive pleading. 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. However, amendments may be subject to limitations, such as the timeliness of the amendment or the potential for prejudice to the opposing party. At the federal level, Rule 15 of the Federal Rules of Civil Procedure provides similar provisions for amending pleadings, with a focus on allowing amendments to facilitate decision on the merits rather than on procedural technicalities, subject to the discretion of the federal court to deny an amendment if it results in undue delay, prejudice, or is futile.