Congress's role and operation in national politics is fundamentally shaped by the design and structure of the governing institutions 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. Experience with the consolidated power of King George III had led them to believe that "the accumulation of legislative, executive, and judicial powers in the same hands … [was] the very definition of tyranny." These objectives were achieved institutionally through the constitutional separation of powers. 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 elected branches have separate, independent bases of authority, and specific safeguards prevent any of the branches from gaining undue influence over another. The constitutional structure does not, however, insulate the branches from each other. While the design of the Constitution aims to prevent the centralization of power through separation, 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. The President has the power to veto legislation; the Senate must approve executive and judicial nominations made by the President; the judiciary has the power to review actions of Congress or the President; and Congress may, by supermajority, remove judges or the President from power. This report provides an overview of separation of powers. It first reviews the philosophical and political origins of the doctrine. Then it surveys the structure of separation of power in the Constitution. It next discusses the consequences of the system, for both the institutions and for individual political actors. Finally, there is a discussion of separation of powers in the context of contemporary politics. The American system of separation of powers is not the most common arrangement of democratic institutions in the modern world. Most modern democracies are parliamentary systems, in which the legislative branch is sovereign, and the executive has no independent constitutional base of authority, instead being chosen by the legislature. Indeed, even when the United States has participated in the construction of democratic governments in the 20 th and 21 st centuries—in Germany and Japan after World War II; in Iraq in the first decade of the 21 st century—the chosen structure has been parliamentary, not an American system of separated powers. This is perhaps, in part, because experience has suggested mass democracy and legislative supremacy—two (at the time) untested concepts that concerned the framers of the U.S. Constitution in the 18 th century—to be satisfactory arrangements for stable democratic governing. There is also a widely held contemporary belief that modern democracy is inherently based on political parties and that parliamentary systems produce stronger parties. Neither of those prospects would have sat well with the framers of the American Constitution, who were skeptical of political parties. Finally, parliamentary systems are often encouraged for fledgling democracies because they tend to produce unified governments that can relatively easily legislate and implement policy without difficulty, allowing for smoother government functioning during the dangerous and unstable early stages of a new national governing system. At their core, parliamentary systems are based on contested elections followed by unified party control of the powers of government. This is quite emphatically not the case with the American system. Our constitutional system is based on contested elections followed by separated control of the powers of governing. The system, by design, produces conflict. The constitutional structure of separation of powers invites conflict between the branches, particularly between Congress and the President. The electoral structure of the federal government provides not only separate bases of authority, but also different bases of authority for political actors, as well as different time horizons. Likewise, the assignment of powers under the Constitution is not only overlapping, but also somewhat vague, creating inter-branch contests for power across many key functions of the government. Finally, numerous questions of authority are not even addressed by the Constitution. Although each branch has strong incentives to protect its prerogatives, in many cases individual political actors have incentives that run counter to their institutional affiliation. In particular, political actors will often, quite reasonably, place the short-term achievement of substantive policy goals ahead of the long-term preservation of institutional power for their branch of government. Likewise, partisan or ideological affiliations will place political actors at cross purposes, where they will be forced to choose between those affiliations and their branch affiliation. Such anti-branch incentives are important contours to consider for political actors seeking to increase the power of their own branch. In order to fully illuminate the contemporary implications of our separation of powers system, it is helpful to understand its origins. The structure of the Constitution reflects the collective preferences of the state delegates who drafted it in 1787. These preferences were chiefly shaped by two things: the political philosophy of the colonial Americans, and their actual political experiences as English colonists. The fingerprints of both can be found throughout the legislative debate at the constitutional convention, the arguments for and against the Constitution during the ratification debate in the states, and, of course, in the text of the Constitution itself. Undoubtedly, the colonists' philosophy also shaped how they understood their colonial interactions with London, and vice versa, making it difficult if not impossible to untangle the degree to which either had primacy in shaping the Constitution. What is clear, however, is that by 1787, the philosophy and the experiences of the framers had created something of a consensus among them about how an optimal government should be structured. The political theory underlying the constitutional separation of powers goes back thousands of years, and traces its development through many eminent philosophers, among them Aristotle, Aquinas, Machiavelli, Locke, and Montesquieu. Virtually all of these philosophers were living under non-democratic systems of government, or systems of very limited democracy that did not feature separation of powers. Therefore, much of their writing is either normative—what should such a system look like?—or applied to non-democratic governing structures. As a result, much of the development of the philosophy ignored the practical problems of establishing a separation of powers democracy, and by the time of the American Revolution, left the framers long on theory but short on practical advice. The conceptual roots of separation of powers are usually attributed to ancient Greek and Roman writers. Aristotle is typically credited with articulating the first conception of government as divided into three basic functions or "powers," which he labeled "deliberative," "magisterial," and "judicial," and which roughly correspond to the contemporary notions of legislative, executive, and judicial roles of government. The ancient philosophers, however, did not conceptualize the possibility of (or benefit of) separating these functions among different officials or institutions. In both Athens and Rome, the various functions were often taken on by single entities. Later philosophers tied this concept of functions to the idea of a "mixed government," in which the interests of society would be balanced through the use of multiple forms of government. This formed the basis of pre-modern English government for centuries: a hereditary monarchy, limited in its authority by a legislature consisting of elements of aristocracy (House of Lords) and democracy (House of Commons). While the "mixed government" system did not separate functions by institution, it did promote the idea that liberty for all could be enhanced by blocking any individual or entity from dominating government. Since these classes of society were rigid, their interest were perpetual and often in conflict. By combining them in the various forms into the government, none would gain the power necessary to dominate the others. The modern conception of separation of powers developed largely among 17 th and 18 th century Enlightenment thinkers. Although many writers were active in this area, John Locke and Montesquieu are usually given credit for articulating the philosophy. In the Second Treatise on Government , Locke argues that a division between the legislative and executive powers is fundamentally necessary to secure the liberty of the people. If the two functions are fused into a single person or entity, the likely result is tyranny. Locke also explains the concept of a "mixed government," in which multiple forms of governing—monarchy, oligarchy, democracy—are simultaneously used. The fully formed conception of separation of powers as understood by the framers of the Constitution is credited to Montesquieu. In The Spirit of Laws , Montesquieu identifies three powers of the government: legislative, executive, and judiciary. He then argues for their placement in the hands of different people or entities: When the legislative and executive power are united in the same person, or in the same body of magistrates, there can be no liberty, because apprehensions may arise, lest the monarch or Senate should enact tyrannical laws, to execute them in a tyrannical manner. Again there is no liberty, if the judiciary power be not separated from the legislative and executive. Were it joined with the legislative, the life and liberty of the subject would be exposed to arbitrary control, for the judge would then be the legislator. Were it joined to the executive power, the judge might behave with violence and oppression. There would be an end of every thing, were the same man, or the same body, whether of the nobles or the people, to exercise those three powers, that of enacting laws, that of executing the public resolutions, and of trying the causes of individuals. Montesquieu also provides the basis for the concept of checks and balances, writing that the executive power and legislative power should be "restrained" by each other. For Montesquieu, the critical executive restraint on the legislature is the veto; the "power of rejecting" legislation. The critical restraint of the legislature is the annual power of the purse, for if "the executive power [were] to determine the raising of public money … liberty would be at an end."