The House of Windsor is the reigning royal house of the United Kingdom and the other Commonwealth realms. In 1901, a line of the House of Saxe-Coburg and Gotha (itself a cadet branch of the House of Wettin) succeeded the House of Hanover to the British monarchy with the accession of King Edward VII, son of Queen Victoria and Prince Albert of Saxe-Coburg and Gotha. In 1917, the name of the British royal house was changed from the German Saxe-Coburg and Gotha to the English Windsor because of anti-German sentiment in the United Kingdom during the First World War. There have been five British monarchs of the House of Windsor since then: George V, Edward VIII, George VI, Elizabeth II, and Charles III. The children and male-line descendants of Queen Elizabeth II and Prince Philip also genealogically belong to the House of Oldenburg since Philip belonged to the Glücksburg branch of that house.

The monarch is head of state of fifteen sovereign states. These are the United Kingdom, Antigua and Barbuda, Australia, the Bahamas, Belize, Canada, Grenada, Jamaica, New Zealand, Papua New Guinea, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Solomon Islands, and Tuvalu. As well as these separate monarchies, there are also three Crown Dependencies, fourteen British Overseas Territories and two small associated states of New Zealand: the Cook Islands and Niue.