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 Kentucky, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Kentucky Rules of Civil Procedure (CR). Rule 15 of the CR allows parties to amend their pleadings, which includes complaints, answers, and replies. A party may amend its pleading once as a matter of course within 20 days after it is served, or if the pleading is one to which a responsive pleading is required, the party may amend within 20 days after service of the responsive pleading. Beyond this, amendments require either consent from the opposing party or leave from the court, which shall be freely given when justice so requires. This is to ensure that cases are decided on their merits rather than on technicalities. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 similarly allows amendments to pleadings, with the philosophy of permitting amendments to facilitate decision on the merits. Both state and federal rules recognize that circumstances may change or come to light as a case progresses, and the rules provide flexibility for parties to assert new claims or defenses, subject to the court's discretion and potential concerns such as prejudice to the opposing party or undue delay.