That non-statutory executive powers are subject to judicial review is beyond doubt. But current judicial practice challenges prevailing theories of judicial review and raises a host of questions about the nature of official power and action. This is particularly the case for official powers not associated with the Royal Prerogative, which have been argued to comprise a “third source” of governmental authority. Looking at non-statutory powers directly, rather than incidentally, stirs up the intense but ultimately inconclusive debate about the conceptual basis of judicial review in English law. This provocative book argues that modern judges and scholars have neglected the very concepts necessary to understand the supervisory jurisdiction and that the law has become more complex than it needs to be. If we start from the concept of office and official action, rather than grand ideas about parliamentary sovereignty and the courts, the central questions answer themselves.