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 South Carolina, the rules governing the amendment of pleadings in civil lawsuits are outlined in the South Carolina Rules of Civil Procedure (SCRCP). Rule 15 of the SCRCP allows a party to amend its pleadings, and it is generally liberal in permitting such amendments. A plaintiff may amend the complaint to assert new claims, and a defendant may amend their answer to assert new defenses. Amendments may be made 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(b), (e), or (f), whichever is earlier. Otherwise, amendments require either consent from the opposing party or leave of court, which the court should freely give when justice so requires. However, amendments may be denied if they cause undue delay, prejudice to the opposing party, or are futile. At the federal level, the Federal Rules of Civil Procedure (FRCP) also allow for amendments. Rule 15 of the FRCP is similar to South Carolina's rule, providing parties with the ability to amend pleadings with the court's leave or the opposing party's written consent, and the court should freely give permission when justice so requires. Both state and federal rules aim to ensure that cases are decided on their merits rather than on technicalities related to pleadings.