Congress’s role and operation in national politics is fundamentally shaped by the design and structure of the governing institution in the Constitution. One of the key principles of the Constitution is separation of powers. The doctrine is rooted in a political philosophy that aims to keep power from consolidating in any single person or entity, and a key goal of the framers of the Constitution was to establish a governing system that diffused and divided power. These objectives were achieved institutionally through the design of the Constitution. The legislative, executive, and judicial branches of the government were assigned distinct and limited roles under the Constitution, and required to be comprised of different political actors. The constitutional structure does not, however, insulate the branches from each other. While the design of the Constitution aims, through separation, to prevent the centralization of power, it also seeks the same objective through diffusion. Thus, most powers granted under the Constitution are not unilateral for any one branch; instead they overlap. This book provides an overview of separation of powers. It reviews the philosophical and political origins of the doctrine; surveys the structure of separation of power in the Constitution; discusses the consequences of the system, for both the institutions and for individual political actors; and provides a discussion of separation of powers in the context of contemporary politics.