William I (about 1027 – 9 September 1087), better known as William the Conqueror, was Duke of Normandy from 1035 and King of England from late 1066 to his death. William is sometimes also referred to as William II in relation to his position as the second Duke of Normandy of that name. In particular, 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, Flemings, and Frenchmen to victory over the English forces of King Harold Godwinson (who died in the conflict) 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 enormity 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. More controversial are possible changes in law, royal administration, trade, agriculture, the peasantry, women's roles and rights, and education.