In Westminster constitutions based on the supremacy of Parliament there remain a number of important governmental powers derived purely from royal prerogatives. They can be potent political weapons allowing the executive government to act without democratic accountability. Some reserve powers are exercised by the monarch or vice-regal representative – and not without controversy in some instances. Most prerogative powers are vested in Cabinet and in ministers responsible for entering, ratifying and withdrawing from treaties, declaring war and dispatching armed forces personnel to theatres of war, and dealing with British subjects in directly ruled Crown overseas territories. Walter Bagehot is invariably the primary authority cited on the role of the sovereign in a constitutional monarchy even though his views did not accurately reflect reality in 1867, and may still disguise reality today. The executive frequently finds it useful and necessary – indeed ‘efficient’ in Bagehot’s terms – to invoke prerogative powers. The Chilcot Iraq inquiry; litigation concerning involuntary removal of British subjects from Diego Garcia for a US military base; and the role or prerogative in the Brexit decision-making are among the cases studies in this chapter.