10 Downing Street, colloquially known in the United Kingdom as "Number 10", is the headquarters of Her Majesty's Government and the official residence and office of the First Lord of the Treasury, an office now invariably held by the Prime Minister.

Situated on Downing Street in the City of Westminster, London, Number 10 is one of the most famous addresses in the United Kingdom and the world. Almost three hundred years old, the building contains about one hundred rooms. There is a private residence on the third floor and a kitchen in the basement. The other floors contain offices and numerous conference, reception, sitting and dining rooms where the Prime Minister works, and where government ministers, national leaders and foreign dignitaries are met and entertained. There is an interior courtyard and, in the back, a terrace overlooking a garden of 0.5 acres (2,000 m²). Adjacent to St. James's Park, Number 10 is near the Houses of Parliament, and Buckingham Palace, the official London residence of the British Monarch.

Downing Street in London, England has for over two hundred years housed the official residences of two of the most senior British cabinet ministers: the First Lord of the Treasury, an office now synonymous with that of Prime Minister of the United Kingdom, and the Second Lord of the Treasury, an office held by the Chancellor of the Exchequer. The Prime Minister's official residence is 10 Downing Street; the Chancellor's official residence is next door at Number 11. The Government's Chief Whip has an official residence at Number 9.

Downing Street is located in Whitehall in central London, a few minutes' walk from the Houses of Parliament and a little farther from Buckingham Palace. The street was built in the 1680s by Sir George Downing (1632–1689) on the site of a mansion called Hampden House. The houses on the west side of the street were demolished in the nineteenth century to make way for government offices, now occupied by the Foreign and Commonwealth Office.

David William Donald Cameron (pronunciation: /ˈkæmərən/; born 9 October 1966) is the Prime Minister of the United Kingdom, First Lord of the Treasury, Minister for the Civil Service and Leader of the Conservative Party. He represents Witney as its Member of Parliament (MP).

Cameron studied Philosophy, Politics and Economics (PPE) at Oxford, gaining a first class honours degree. He then joined the Conservative Research Department and became Special Adviser to Norman Lamont, and then to Michael Howard. He was Director of Corporate Affairs at Carlton Communications for seven years.

He was defeated in his first candidacy for Parliament at Stafford in 1997, but was elected in 2001 as the Member of Parliament for the Oxfordshire constituency of Witney. He was promoted to the Opposition front bench two years later, and rose rapidly to become head of policy co-ordination during the 2005 general election campaign. With a public image of a youthful, moderate candidate who would appeal to young voters, he won the Conservative leadership election in 2005.