division of the legislative, executive, and judicial (judiciary) functions of government among separate and independent bodies. Such a separation, it has been argued, limits the possibility of arbitrary excesses by government, since the sanction of all three branches is required for the making, executing, and administering of laws.

The doctrine may be traced to ancient and medieval theories of mixed government, which argued that the processes of government should involve the different elements in society such as monarchic, aristocratic, and democratic interests. The first modern formulation of the doctrine was that of the French writer Montesquieu (Montesquieu, Charles-Louis de Secondat, baron de La Brède et de) in De l'esprit des lois (1748), although the English philosopher John Locke had earlier argued that legislative power should be divided between king and Parliament.

Montesquieu's argument that liberty is most effectively safeguarded by the separation of powers was inspired by the English constitution, although his interpretation of English political realities has since been disputed. His work was widely influential, most notably in America, where it profoundly influenced the framing of the Constitution. The U.S. Constitution (Constitution of the United States of America) further precluded the concentration of political power by providing staggered terms of office in the key governmental bodies.

Modern constitutional systems show a great variety of arrangements of the legislative, executive, and judicial processes, and the doctrine has consequently lost much of its rigidity and dogmatic purity. In the 20th century, and especially since World War II, governmental involvement in numerous aspects of social and economic life has resulted in an enlargement of the scope of executive power. Some who fear the consequences of this for individual liberty have favoured establishing means of appeal against executive and administrative decisions (for example, through an ombudsman), rather than attempting to reassert the doctrine of the separation of powers.