Whether the Court could review the District Court's denial of the aides' immunity defense before final judgment.
Holding
Yes. The denial of an immunity defense was immediately reviewable under the collateral-order doctrine.
Reasoning
The Court treated its jurisdiction as controlled by its companion decision in Nixon v. Fitzgerald. A denial of immunity may be reviewed before final judgment because immunity is meant to protect officials not merely from ultimate liability, but from the burdens of litigation itself.