This chapter explores the different ways of approaching judicial review, and makes some initial criticisms of the historical ‘ultra vires debate’ of the 1990s and 2000s. It argues for a return to the concept of jurisdiction (and jurisdictional error) as a central category of the law, and for a focus on non-statutory executive powers as a primary case for and from which to build a theory of the supervisory jurisdiction. While legislative intention is important, wherever it is relevant, it is not relevant where the official powers in question obviously derive from rules of competence outside of legislation. Further, common law rules of conduct are always central to judicial review in the context of statutory grants of power. This insight is conducive to a simpler and more powerful conception of judicial review based around a common law doctrine of ultra vires.