This chapter provides some more detail on the rules of competence and rules of conduct that empower officials to act as such and constrain the scope of their action. The logic of empowerment, and the fundamental distinction between ability and permission, provides a profound insight into the two traditional ‘limbs’ of the ultra vires rule – the ‘narrow’ limb concerned with competence, jurisdiction, or vires in the strict sense, and the ‘broad limb’ concerned with the manner, form, and purpose of an exercise of a competence that the official clearly does have. Not only does this provide a simple and powerful explanation for the ultra vires review of non-statutory executive powers; it provides the foundation for a general theory of judicial review based in the courts’ inherent (common law) jurisdiction to police the bounds of, and constraints upon, officials’ vires – whether statutory or non-statutory.