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 Jersey, the rules governing the amendment of pleadings in civil lawsuits are found in the New Jersey Court Rules, specifically Rule 4:9. This rule allows parties to amend their pleadings, which includes complaints, answers, and replies, to assert new claims or defenses. Amendments may be made once as a matter of course within 35 days after service of the pleading to be amended, or if the pleading is one to which a responsive pleading is required, 35 days after service of the responsive pleading or 35 days after service of a motion under Rule 4:6-2, whichever is earlier. Beyond this, parties must seek consent from the opposing party or leave of court. The court is generally liberal in granting permission to amend, especially when the case is in its early stages and the amendment would not cause undue prejudice to the other party. However, as the case progresses, the court may consider factors such as the delay in seeking amendment, the reason for the delay, and potential prejudice to the opposing party. At the federal level, the Federal Rules of Civil Procedure, specifically Rule 15, govern the amendment of pleadings. Rule 15(a) 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 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. After this period, the party must seek consent from the opposing party or leave of court, which should be freely given when justice so requires. Both New Jersey state courts and federal courts encourage amendments that will help in resolving all the matters in dispute between the parties.