When a legal claim becomes moot while awaiting appellate review, the established practice is for the federal appeals court to reverse or vacate the judgment below and to remand the case to the district court with an instruction to dismiss the action. That consequence is because a moot case does not qualify as a "case or controversy" under Article III; due to the lack of jurisdiction, federal courts have no power to consider the merits of a constitutionally moot case. Cases may be rendered moot because of a change in the status of the parties or in the law, or because of an act of one of the parties that dissolves the controversy. Exceptions to the Mootness Doctrine

The Supreme Court has recognized several exceptions to the mootness doctrine that, if found to apply to a case, would permit federal court adjudication of the dispute.
A case pending before a federal court may at some point in the litigation process lose an element of justiciability and become "moot." Mootness may occur when a controversy initially existing at the time the lawsuit was filed is no longer "live" due to a change in the law or in the status of the parties involved, or due to an act of one of the parties that dissolves the dispute. When a federal court deems a case to be moot, the court no longer has the power to entertain the legal claims and must dismiss the complaint. However, the U.S. Supreme Court over time has developed several exceptions to the mootness doctrine. This report provides a general overview of the doctrine of "mootness," as the principle is understood and used by federal courts to decide whether to dismiss certain actions for lack of jurisdiction.