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 Wisconsin, the rules of civil procedure allow parties to amend their pleadings, which include complaints, petitions, and answers. According to Wisconsin Statutes Section 802.09, a party may amend its pleading once as a matter of course within a specified time frame—either before a responsive pleading is served or, if the pleading is one to which no responsive pleading is permitted and the action has not been placed on the trial calendar, within 20 days after it is served. Beyond this, parties must seek the court's permission to amend, and the court is directed to freely give permission when justice so requires. This is consistent with the Federal Rules of Civil Procedure Rule 15, which also encourages liberality in granting amendments to pleadings. However, amendments may be subject to limitations based on factors such as undue delay, bad faith, dilatory motive, prejudice to the opposing party, or futility of the amendment. The timing and the substance of the proposed amendment can affect the court's decision to grant or deny an amendment.