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 Hampshire, the rules governing the amendment of pleadings in civil lawsuits are outlined in the New Hampshire Rules of Civil Procedure. Rule 15 allows a party to amend its pleadings, which includes a plaintiff amending a complaint or a defendant amending an answer. Amendments may be made once as a matter of course within a certain time frame (for example, for a plaintiff, before a responsive pleading is served, or for a defendant, within 20 days after serving the original answer). Beyond this, amendments require either consent from the opposing party or leave from the court, which the court should freely give when justice so requires. However, the court may deny an amendment if it finds reasons such as undue delay, bad faith, dilatory motive, repeated failure to cure deficiencies by amendments previously allowed, undue prejudice to the opposing party, or futility of the amendment. At the federal level, the Federal Rules of Civil Procedure also provide similar provisions under Rule 15, allowing for amendments to pleadings with the same general principles of liberality and limitations based on fairness and procedural efficiency.