A federal court of appeals granted habeas relief on a theory that the petitioner had not properly presented and that the parties had not litigated. The Supreme Court vacated the judgment. Courts ordinarily decide the questions the parties place before them, and a court that identifies a potentially dispositive unbriefed issue must provide fair notice and an opportunity to respond before relying on it. The departure from the party-presentation principle was especially consequential in habeas litigation, where statutory exhaustion, procedural-default, and deference rules depend on the claim actually asserted. The case was remanded for consideration through an adversarial process.