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 Ohio, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Ohio Rules of Civil Procedure (ORCP) Rule 15, a party may amend its pleading once as a matter of course within 28 days after it is served, or if the pleading is one to which a responsive pleading is required, within 28 days after service of the responsive pleading or 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 of court, which the court should freely give when justice so requires. However, amendments may be denied if they cause undue delay, prejudice to the opposing party, or are futile. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 operates similarly, allowing one amendment as a matter of course within 21 days after serving the pleading, or if the pleading is one that requires a response, within 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. Subsequent amendments require the opposing party's consent or the court's leave. The court should freely give leave when justice so requires, but considerations of undue delay, bad faith, or dilatory motive on the part of the movant, repeated failure to cure deficiencies by amendments previously allowed, undue prejudice to the opposing party, and futility of amendment are taken into account.