In the United States legal system, a plaintiff formally begins a civil action when he files a complaint, which names a defendant, in court. A complaint is a short and plain statement of the facts surrounding the controversy. An amended complaint is a revision of the original complaint, previously filed by the plaintiff. A defendant usually has 21 days, from the date on which he is served with the summons and complaint, to officially file his response or answer in court.

There are many reasons why a plaintiff may want to file an amended complaint. A plaintiff may have subsequently discovered additional facts about the lawsuit, since the time he filed the original complaint. In addition, a plaintiff may wish to amend his complaint to include additional causes of action, or requests for relief, or to name additional parties as defendants to the action.

In some cases, a plaintiff will need to file an amended complaint because a court may have granted a defendant’s motion to dismiss the plaintiff's complaint. This may happen if it failed to state a legal claim upon which relief can be granted, or if the allegations made were too vague. In situations where a plaintiff’s original complaint was legally defective, rather than dismissing the plaintiff’s case entirely with no further recourse, many courts will allow the plaintiff to file an amended complaint in order to correct the inadequacies. Most jurisdictions in the United States follow the federal rules of civil procedure, which govern the manner in which a complaint may be amended.

Rule 15 of the federal rules of civil procedure provides that a plaintiff may amend his complaint as a matter of course, if the defendant has not filed an answer to the original complaint. If the defendant has filed his answer to the original complaint, the plaintiff may amend his complaint only by consent of the defendant, or with the court’s permission. Most courts freely allow a plaintiff to amend his complaint unless the amendment would substantially prejudice the defendant.

An amended complaint must be also be served upon the defendant. The defendant is then allowed to respond to it within the response time remaining on the original complaint. This is usually 21 days from the date on which it was served, or 10 days from the date of service of the amended complaint, whichever time period is longer.