A queen regnant (plural: queens regnant) is a female monarch, equivalent in rank and title to a king, who reigns in her own right over a realm known as a "kingdom"; as opposed to a queen consort, who is the wife of a reigning king; or a queen regent, who is the guardian of a child monarch and rules temporarily in the child's stead, be it de jure in sharing power or de facto in ruling alone. A princess regnant is a female monarch who reigns in her own right over a "principality"; an empress regnant is a female monarch who reigns in her own right over an "empire".

A queen regnant possesses and exercises sovereign powers, whereas a queen consort or queen regent shares her spouse's and/or child's rank and titles but does not share the sovereignty of her spouse or child. The husband of a queen regnant traditionally does not share the queen regnant's rank, title, or sovereignty. However, the concept of a king consort or prince consort is not unheard of in both contemporary and classical periods.

A queen dowager or empress dowager is the widow of a king or emperor; a queen mother is a queen dowager who is also the mother of a reigning sovereign.

Elizabeth II (Elizabeth Alexandra Mary; born 21 April 1926) is Queen of the United Kingdom and 14 other Commonwealth realms.Elizabeth was born in Mayfair, London, as the first child of the Duke and Duchess of York (later King George VI and Queen Elizabeth). Her father acceded to the throne in 1936 upon the abdication of his brother, King Edward VIII, making Elizabeth the heir presumptive. She was educated privately at home and began to undertake public duties during the Second World War, serving in the Auxiliary Territorial Service. In November 1947, she married Philip Mountbatten, a former prince of Greece and Denmark, and their marriage lasted 73 years until Philip's death in 2021. They had four children: Charles, Prince of Wales; Anne, Princess Royal; Prince Andrew, Duke of York; and Prince Edward, Earl of Wessex.

When her father died in February 1952, Elizabeth—then 25 years old—became queen regnant of seven independent Commonwealth countries: the United Kingdom, Canada, Australia, New Zealand, South Africa, Pakistan, and Ceylon, as well as Head of the Commonwealth. Elizabeth has reigned as a constitutional monarch through major political changes such as the Troubles in Northern Ireland, devolution in the United Kingdom, the decolonisation of Africa, and the United Kingdom's accession to the European Communities and withdrawal from the European Union. The number of her realms has varied over time as territories have gained independence, and as some realms have become republics. Her many historic visits and meetings include a state visit to the Republic of Ireland in 2011 and visits to or from five popes.

Significant events have included the Queen's coronation in 1953 and the celebrations of her Silver, Golden, Diamond and Platinum jubilees in 1977, 2002, 2012 and 2022 respectively. Elizabeth is the longest-lived and longest-reigning British monarch, the longest-serving female head of state in history, the oldest living and longest-reigning current monarch, and the oldest and longest-serving incumbent head of state. Elizabeth has occasionally faced republican sentiment and press criticism of the royal family, particularly after the breakdown of her children's marriages, her annus horribilis in 1992, and the death in 1997 of her former daughter-in-law Diana, Princess of Wales. However, support for the monarchy in the United Kingdom has been and remains consistently high, as does her personal popularity.