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 New York State, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. Under the New York Civil Practice Law and Rules (CPLR), a party may amend its pleading once without permission within 20 days after serving it, or if a response is required, within 20 days after service of a responsive pleading or service of a motion for summary judgment. Beyond this, or if the amendment would change the parties to the action, the party must seek the court's permission, which is to be freely given when justice so requires. However, amendments may be subject to limitations, such as the statute of limitations or prejudice to the opposing party. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 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 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, which should be freely given when justice so requires. Both New York State and federal courts encourage amendments to serve the interests of justice, but they also consider factors such as undue delay, bad faith, or dilatory motive on the part of the movant.