When Congress delegates the power to regulate to a federal agency, it often also provides for judicial review of that agency’s actions. Reviewing courts are generally empowered to “set aside” agency action that is arbitrary and capricious or contrary to law. Agency nonacquiescence may arise after a court sets aside agency action because the court disagrees with the agency’s interpretation of law. The agency must determine whether it will conform its future actions to that court’s interpretation of law—acquiescence—or whether it will continue to apply its preferred interpretation in future actions—nonacquiescence. Nonacquiescence is possible primarily, although not exclusively, in situations when a reviewing court overturns a decision made by an agency through adjudication rather than rulemaking. Nonacquiescence raises foundational questions about which branch of government (the executive or the judiciary) has the ultimate authority to interpret federal statues and about the federal judiciary’s authority to issue decisions that bind future agency actions, not just the parties before the court.