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 Minnesota, the rules governing the amendment of pleadings in civil lawsuits are outlined in the Minnesota Rules of Civil Procedure. Rule 15.01 allows a party to 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, 20 days after service of the responsive pleading or service of a motion under Rule 12, whichever is earlier. Beyond this, parties may amend their pleadings only with the opposing party's written consent or the court's permission, which the court should freely give when justice so requires. This is consistent with the federal rules, as outlined in the Federal Rules of Civil Procedure Rule 15, which also encourages liberality in amending pleadings to ensure that cases are decided on their merits rather than on technicalities. However, amendments may be denied if they cause undue delay, prejudice to the opposing party, are sought in bad faith, or would be futile. The timing and frequency of amendments are subject to the discretion of the court, and in both state and federal courts, amendments sought on the eve of or during trial are scrutinized more closely to prevent prejudice and ensure fairness to all parties involved.