Various political thinkers have distinguished types of government activity. Montesquieu was the first, however, to urge the creation of three separate institutions or divisions of government - the executive, legislative, and judicial - a distinction that became common in almost all modern constitutions. Some governmental structures, notably that of the United States, are based on the principle of separation of powers at nearly every level. Executive, legislative, and judicial powers are divided into three branches of government, creating a system of checks and balances among them and helping to protect citizens from arbitrary and capricious actions on the part of any of the three branches. Such protection is crucial in the area of civil rights - those constitutionally guaranteed rights that shield the citizen from tyrannical actions by government. Often, in times of grave national emergency, when the central government needs more power, the public is willing to grant it. The executive branch usually predominates at such time.

Proponents of the separation of powers bring an additional argument in its favor: they point out that the system diminishes the influence of special-interest groups over any one branch of government or over the government as a whole. It is difficult for even the strongest faction to dominate a government in which the executive is elected by the entire population, members of the legislature represent different geographical constituencies, and the judges are appointed by the executive with the approval of the legislature.

Not all states, of course, have such clear divisions of government, nor do divisions necessarily guarantee personal liberties. Parliamentary democratic systems, for example, tend to merge legislative and executive functions yet control the exercise of power by constitutional methods of sharing it. Authoritarian states may, however, be constitutionally bound to have separate organs of government yet actually concentrate power in the executive.