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The Cabinet is the collective decision-making body of Her Majesty's Government in the United Kingdom. It is composed of the Prime Minister and, in recent decades, from eighteen to twenty-four Cabinet Ministers. These 'Ministers of the Crown' constitute the most senior level of government officials, and are assigned key executive roles within the Government. They are selected primarily from the elected membership of the House of Commons (and, less frequently, from the membership of the House of lords) by the Prime Minister. Cabinet Ministers are heads of government departments, mostly with the office of 'Secretary of State for X', where 'X' may be, for instance, 'Defence', 'Foreign and Commonwealth Affairs, 'the Home Department', etc. The collective coordinating function of the Cabinet is reinforced by the statutory position that all Secretaries of State jointly hold the same office and exercise the same governmental powers.

Traditional constitutional theory holds that the Cabinet is the ultimate decision-making body of the executive within the system of government present in the United Kingdom. The nineteenth century constitutional theorist Walter Bagehot, for instance, described the Cabinet as the 'efficient secret' of the British political system (see, for example, Bagehot's 'The English Constitution'). However, some theorists and commentators now argue that the Cabinet's political and decision-making authority is no longer what it once was, having been supplanted in recent decades by the growing power and authority of the office of the Prime Minister.

The Cabinet is technically the executive committee of Her Majesty's Privy Council, a historic body with legislative, judicial and executive functions. The decisions of this body, whose sizeable membership includes members of the opposition party in Parliament, are generally implemented either under the existing powers of individual government departments, or by an order in council.

See also Minister, Privy Council, order in council.