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 Nevada, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Nevada Rules of Civil Procedure (NRCP). According to NRCP 15(a), a party may 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, 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. Beyond this, a party must seek the opposing party's written consent or the court's leave to amend, which the court should freely give when justice so requires. This is consistent with the Federal Rules of Civil Procedure (FRCP) Rule 15, which also encourages courts to grant leave to amend when justice requires. There are limitations, however, such as when an amendment would cause undue delay, prejudice to the opposing party, or is sought in bad faith. Amendments to pleadings may also be limited by statutes of limitations, as new claims must still be timely. In federal courts, the FRCP applies, and the principles are similar to Nevada's rules, emphasizing the flexibility to amend pleadings to promote the determination of cases on their merits.