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 Maryland, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to the Maryland Rules of Civil Procedure, specifically Rule 2-341, a party may amend its pleadings once as a matter of course within 30 days after the pleading is served, or at any time before the opposing party files a responsive pleading. After this period, or if the amendment would introduce a new cause of action or defense after the statute of limitations has expired, the party must seek the court's permission to amend, which the court may grant if it finds that the amendment would not unfairly prejudice the other party or significantly delay the proceedings. At the federal level, Rule 15 of the Federal Rules of Civil Procedure similarly allows a party to 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, parties must seek the court's permission or obtain written consent from the opposing party. The courts generally favor granting leave to amend in the interest of justice, unless there is a good reason to deny such as undue delay, bad faith, or dilatory motive on the part of the movant.