Federal district courts adopted a policy requiring most in-custody defendants to appear in nonjury proceedings wearing full restraints, including handcuffs connected to a waist chain and leg shackles. Four defendants challenged the policy, but their individual criminal cases ended before the Court of Appeals ruled. The Ninth Circuit treated the challenge as a functional class action and reached the merits. The Supreme Court held that the case was moot and vacated the judgment. Federal courts may decide only an actual, concrete dispute that remains live through every stage of review. The civil class-action cases allowing some claims to continue after a named plaintiff’s individual claim becomes moot did not create a freestanding exception for criminal defendants seeking prospective relief in their separate prosecutions. Nor did the possibility that two respondents might commit future crimes and again be detained satisfy the capable-of-repetition exception. The Court declined to decide whether the restraint policy was constitutional, vacated the Ninth Circuit’s judgment, and remanded with instructions to dismiss the case as moot.