William the Conqueror (French: Guillaume le Conquérant, Guillaume le Bâtard) (circa 1028 – 9 September 1087), also known as William I of England, was the King of England from Christmas, 1066 until his death. He was also Duke of Normandy from 3 July 1035 until his death. Before his conquest of England, he was known as William the Bastard because of the illegitimacy of his birth.

To press his claim to the English crown, William invaded England in 1066, leading an army of Normans, Bretons, Flemish people, and Frenchmen (from Paris and Île-de-France) to victory over the English forces of King Harold Godwinson at the Battle of Hastings, and suppressed subsequent English revolts in what has become known as the Norman Conquest. His reign, which brought Norman-French culture to England, had an impact on the subsequent course of England in the Middle Ages. The details of that impact and the extent of the changes have been debated by scholars for over a century. In addition to the obvious change of ruler, his reign also saw a programme of building and fortification, changes to the English language, a shift in the upper levels of society and the church, and adoption of some aspects of continental church reform.