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 Oklahoma, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to the Oklahoma Pleading Code, 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, within 20 days after service of the responsive pleading. Beyond this, 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 subject to limitations, such as not causing undue delay, prejudice to the opposing party, or futility of the amendment. At the federal level, the Federal Rules of Civil Procedure (FRCP) Rule 15 similarly allows parties to amend their pleadings with the court's permission, which should be freely given when justice requires, and provides specific time frames for doing so. Both Oklahoma state and federal courts 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.