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 Tennessee, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Tennessee Rules of Civil Procedure. Rule 15.01 allows a party to amend its pleadings once as a matter of course within 30 days after the pleading is served, or if the pleading is one to which a responsive pleading is required, 30 days after service of the responsive pleading or service of a motion under Rule 12.02, whichever is earlier. Beyond this, parties may amend their pleadings only with the opposing party's written consent or the court's leave, which the court should freely give when justice so requires. At trial, the court may permit pleadings to be amended to conform to the evidence. Similarly, the Federal Rules of Civil Procedure (Rule 15) provide for the amendment of pleadings, with the philosophy that cases should be decided on their merits and not on the technicalities of pleadings. Amendments before trial are allowed with the court's discretion, and during trial, amendments to conform to the evidence are also permitted. Both sets of rules aim to ensure that the actual merits of the case can be reached without undue prejudice to the parties involved.