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 Dakota, the rules governing the amendment of pleadings in civil lawsuits are outlined in the South Dakota Codified Laws (SDCL) and are similar to the Federal Rules of Civil Procedure. Under SDCL 15-6-15(a), a party may amend its pleading once as a matter of course within 30 days after serving it, or if the pleading is one to which a responsive pleading is required, within 30 days after service of the responsive pleading or service of a motion under Rule 12(b), (e), or (f), whichever is earlier. Beyond this, amendments are allowed only with the opposing party's written consent or the court's leave, which the court should freely give when justice so requires. Amendments to pleadings may be necessary to correct errors, add claims or defenses, or to conform to the evidence. However, the court may deny an amendment if it results in undue delay, prejudice to the opposing party, or is futile. The Federal Rules of Civil Procedure Rule 15 operates similarly, allowing parties to amend their pleadings with the court's permission, which should be freely given when justice requires. Both 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.