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 Iowa, as in other states, the rules of civil procedure allow litigants to amend their pleadings, which include complaints, petitions, and answers. According to Iowa Rule of Civil Procedure 1.402, a party may amend its pleading once as a matter of course within a certain time frame—typically before a responsive pleading is filed or if the pleading is one to which no responsive pleading is permitted and the action has not been placed upon the trial calendar. Beyond this, amendments may be allowed with the opposing party's written consent or by leave of the court, which should be freely given 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, Rule 15 of the Federal Rules of Civil Procedure provides similar provisions for amending pleadings, with the philosophy of favoring decisions on the merits rather than on procedural technicalities. Both state and federal rules recognize that circumstances may change as a case progresses, and the ability to amend pleadings ensures that the court can consider the most accurate and complete set of claims and defenses.