A feature common to most governments in different time periods and places is that they possess exceptional powers applicable to emergencies. These authorities are necessary but vulnerable to abuse. Democracies seek to create and restrain such powers through constitutional texts and supplementary legislation. In the UK, lacking a written constitution, many of these executive entitlements have traditionally derived from the Royal Prerogative. This constitutional source comprises a group of residual monarchical authorities that have largely devolved in practice to ministers. As an historical entity the prerogative can only fully be understood in its more recent manifestations through taking a long-term perspective. It has often attracted controversy on the grounds that it is not subject to sufficient levels of what would now be termed accountability. At various points reform has been demanded and sometimes achieved. Since the later decades of the 20th century there has been an erosion of the prerogative, including its emergency powers facets. Parts of it have transferred to a statutory basis, while those elements that remain have become subject to enhanced legal and political constraints. This trend was part of a wider process of constitutional transformation traceable to the 1950s. Early in the twenty-first century the Royal Prerogative, already undergoing change, moved further to the centre of the constitutional reform agenda. This heightened salience arose from altered global security concerns, and associated developments in UK external policy. The Iraq conflict especially encouraged a focus on the powers under which the UK participated in this action. Calls for change, aimed at giving Parliament an enhanced role in war-making and over other matters, followed. Eventually a broad government reform package appeared. A convention regarding parliamentary consultation over engagement in armed combat developed. But features of the prerogative have persisted, including those allowing for government to act in the most perilous of circumstances. A thorough eradication of the Royal Prerogative could require a more complete overhaul of UK governmental arrangements, embodying them in a written constitution.