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 Florida, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Florida Rules of Civil Procedure. Rule 1.190 allows a party to amend its pleadings 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, 20 days after service of a responsive pleading or 20 days after service of a motion under Rule 1.140 (b), whichever is earlier. Beyond this, parties must seek the court's permission to amend, which courts will freely give when justice so requires and the amendment does not prejudice the opposing party. At the federal level, Rule 15 of the Federal Rules of Civil Procedure provides similar provisions, allowing parties to amend their pleadings with the court's leave, which should be freely given when justice so requires, and sets forth the timing and conditions under which amendments may be made. Both state and federal rules aim to ensure that cases are decided on their merits rather than on procedural technicalities, but they also seek to prevent undue delays and prejudice to the opposing party.