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 Maine, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Maine 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 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 21 days after service of a motion under Rule 12(b), (e), or (f), whichever is earlier. 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 results in undue delay, prejudice to the opposing party, or is futile. At the federal level, the Federal Rules of Civil Procedure Rule 15 similarly allows for amendments, with the philosophy that cases should be decided on their merits rather than the technicalities of pleadings. Both state and federal rules recognize that circumstances may change as a case progresses, and the rules are designed to provide flexibility while balancing the interests of fairness and efficiency in the litigation process.