This chapter focuses on jurisdiction, beginning with the distinction between jurisdictional and non-jurisdictional matters. In particular, it considers whether something falls into the ‘jurisdictional’ or ‘merits’ question and how to decide into which category a given matter should be placed. It then examines the power to detain illegal entrants and how to determine whether a particular individual is an illegal entrant. It also discusses jurisdictional and non-jurisdictional errors of law and goes on to review one of the most celebrated cases in modern administrative law: the decision of the House of Lords in Anisminic Ltd v. Foreign Compensation Commission [1969] 2 AC 147. The chapter concludes with an analysis of errors of law as jurisdictional errors; the institutional status of the decision-maker; the nature of the statutory provision; applying statutory criteria to the facts; supervision of the fact-finding process; subjective jurisdictional criteria; and non-compliance with statutory requirements.