Margaret Hilda Thatcher, Baroness Thatcher, LG, OM, PC, FRS (née Roberts; born 13 October 1925) served as Prime Minister of the United Kingdom from 1979 to 1990 and Leader of the Conservative Party from 1975 to 1990. She is the only woman to have held either post.

Born in Grantham in Lincolnshire, United Kingdom, she went to school at Kesteven and Grantham Girls' School in Grantham, where she was head girl in 1942–43. She read chemistry at Somerville College, Oxford and later trained as a barrister. She won a seat in the 1959 general election, becoming the MP for Finchley as a Conservative. When Edward Heath formed a government in 1970, he appointed Thatcher Secretary of State for Education and Science. Four years later, she backed Keith Joseph in his bid to become Conservative Party leader but he was forced to drop out of the election. In 1975 Thatcher entered the contest herself and became leader of the Conservative Party. At the 1979 general election she became Britain's first female Prime Minister.

In her foreword to the 1979 Conservative manifesto, Thatcher had written of "a feeling of helplessness, that a once great nation has somehow fallen behind." She entered 10 Downing Street determined to reverse what she perceived as a precipitate national decline. Her political philosophy and economic policies emphasised deregulation, particularly of the financial sector, flexible labour markets, and the selling off and closing down of state owned companies and withdrawing subsidy to others. Amid a recession and high unemployment, Thatcher's popularity declined, though economic recovery and the 1982 Falklands War brought a resurgence of support and she was re-elected in 1983. She took a hard line against trade unions, survived the Brighton hotel bombing assassination attempt and opposed the Soviet Union (her tough-talking rhetoric gained her the nickname the "Iron Lady"); she was re-elected for an unprecedented third term in 1987. The following years would prove difficult, as her Poll tax plan was largely unpopular, and her views regarding the European Community were not shared by others in her Cabinet. She resigned as Prime Minister in November 1990 after Michael Heseltine's challenge to her leadership of the Conservative Party.

Thatcher's tenure as Prime Minister was the longest since that of Lord Salisbury and the longest continuous period in office since Lord Liverpool in the early 19th century. She was the first woman to lead a major political party in the United Kingdom, and the first of only four women to hold any of the four great offices of state. She holds a life peerage as Baroness Thatcher, of Kesteven in the County of Lincolnshire, which entitles her to sit in the House of Lords.