This chapter introduces the central technical apparatus to describe official action and thus, also, to describe the mechanics of judicial review of official action implicit in the traditional terminology of ‘ultra vires', that is, acting ‘beyond one's powers'. Officials are reposed with powers in virtue of rules of two types, which I call rules of competence and rules of conduct. These are not just labels of convenience, they explain the constitution of office and the inherently legal and limited nature of official vires. They box out the ‘space’ in which an official (i) can and (ii) may act lawfully, which space I call ‘Spielraum'. This concept relates to a modified Hohfeldian schema of jural relations which removes the temptation of "residual liberties" as a valid category of the official legal position.