Constitutional Agency offers a compelling interpretation of the United Kingdom's constitutional order by a figure central to its operation. Drawing on his 2025 Clarendon Law Lectures, Philip Sales (Lord Sales) sets out a vision of the origins, values, and future of the UK constitution animated by the tripartite division of power between the executive, the legislature, and the courts. The constitutional agency these institutions exercise comprises the capacity for action conferred on them by the constitution, and also their part in modifying the constitution itself. Sales shows how the constitutional culture of the UK has changed over time, with attendant changes in the relationship among the institutions as well as in the law itself, but argues that the constitution continues to effectively serve as a mechanism to legitimately mediate the ever-present tensions between collective self-government and individual autonomy. Sales emphasizes that the UK's constitution is a political achievement that should command the loyalty of its citizens, and that the executive, Parliament and the courts for their part should exercise their constitutional agency to maintain that achievement. In doing so responsibly, they inspire a form of constitutional patriotism to sustain the state itself.