The separation of powers principle functionally differentiates several types of state power (usually law-making, adjudication, and execution) and requires these operations of government to be conceptually and institutionally distinguishable and articulated, thereby maintaining the integrity of each. To put this model into practice, government is divided into structurally independent branches to perform various functions (most often a legislature, a judiciary and an administration, sometimes known as the trias politica). When each function is allocated strictly to one branch, a government is described as having a high degree of separation; whereas, when one person or branch plays a significant part in the exercise of more than one function, this represents a fusion of powers. When one branch holds unlimited state power and delegates its powers to other organs as it sees fit, as is the case in communist states, that is called unified power.

Polybius (Histories, Book 6, 11–13) described the Roman Republic as a mixed government ruled by the Roman Senate, Consuls and the Assemblies. Polybius explained the system of checks and balances in detail, crediting Lycurgus of Sparta with the first government of this kind.

During the English Civil War, the parliamentarians viewed the English system of government as composed of three branches – the King, the House of Lords and the House of Commons – where the first should have executive powers only, and the latter two legislative powers. One of the first documents proposing a tripartite system of separation of powers was the Instrument of Government, written by the English general John Lambert in 1653, and soon adopted as the constitution of England for few years during The Protectorate. The system comprised a legislative branch (the Parliament) and two executive branches, the English Council of State and the Lord Protector, all being elected (though the Lord Protector was elected for life) and having checks upon each other.

A further development in English thought was the idea that the judicial powers should be separated from the executive branch. This followed the use of the juridical system by the Crown to prosecute opposition leaders following the Restoration, in the late years of Charles II and during the short reign of James II (namely, during the 1680s).

An earlier forerunner to Montesquieu's tripartite system was articulated by John Locke in his work Two Treatises of Government (1690). In the Two Treatises, Locke distinguished between legislative, executive, and federative power. Locke defined legislative power as having "the right to direct how the force of the commonwealth shall be employed" (Second Treatise, § 143), while executive power entailed the "execution of the laws that are made, and remain in force" (Second Treatise, § 144). Locke further distinguished federative power, which entailed "the power of war and peace, leagues and alliances, and all transactions with all persons and communities without [outside] the commonwealth" (Second Treatise, § 145), or what is now known as foreign policy. Locke distinguishes between separate powers but not discretely separate institutions, and notes that one body or person can share in two or more of the powers. For instance, Locke noted that while the executive and federative powers are different, they are often combined in a single institution (Second Treatise, § 148).

Locke believed that the legislative power was supreme over the executive and federative powers, which are subordinate. Locke reasoned that the legislative was supreme because it has law-giving authority; "[F]or what can give laws to another, must need to be superior to him" (Second Treatise, § 150). According to Locke, legislative power derives its authority from the people, who have the right to make and unmake the legislature. He argues that once people consent to be governed by laws, only those representatives they have chosen can create laws on their behalf, and they are bound solely by laws enacted by these representatives.

Locke maintained that there are restrictions on the legislative power. Locke says that the legislature cannot govern arbitrarily, cannot levy taxes, or confiscate property without the consent of the governed (cf. "No taxation without representation"), and cannot transfer its law-making powers to another body, known as the nondelegation doctrine (Second Treatise, § 142).

The term "tripartite system" is commonly ascribed to French Enlightenment political philosopher Montesquieu, although he did not use such a term but referred to the "distribution" of powers. In The Spirit of Law (1748), Montesquieu described the various forms of distribution of political power among a legislature, an executive, and a judiciary. Montesquieu's approach was to present and defend a form of government whose powers were not excessively centralized in a single monarch or similar ruler (a form known then as "aristocracy"). He based this model on the Constitution of the Roman Republic and the British constitutional system. Montesquieu took the view that the Roman Republic had powers separated so that no one could usurp complete power. In the British constitutional system, Montesquieu discerned a separation of powers among the monarch, Parliament, and the courts of law.