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 Michigan, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Michigan Court Rules (MCR). Specifically, MCR 2.118 allows a party to amend its pleadings once as a matter of course within 14 days after the pleading is served or, if the pleading is one to which a responsive pleading is required, within 14 days after service of the responsive pleading or 14 days after service of a motion under MCR 2.116, whichever is earlier. Beyond this, a party must seek the court's permission to amend, and the court is guided by the principle of allowing amendments to promote justice and facilitate the resolution of the case on its merits, unless there is evidence of undue delay, prejudice to the opposing party, or futility of the amendment. At the federal level, Rule 15 of the Federal Rules of Civil Procedure 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 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. Thereafter, the party may amend only with the opposing party's written consent or the court's leave, which the court should freely give when justice so requires. Both state and federal rules recognize the importance of allowing parties to assert new claims or defenses as the case evolves, while also balancing the interests of fairness and efficiency in the litigation process.