William I (c. 1028 – 9 September 1087), usually known as William the Conqueror and sometimes William the Bastard, was the first Norman King of England, reigning from 1066 until his death in 1087. A descendant of Rollo, he was Duke of Normandy (as William II) from 1035 onward. After a long struggle to establish his power, by 1060 his hold on Normandy was secure, and he launched the Norman conquest of England six years later. The rest of his life was marked by struggles to consolidate his hold over England and his continental lands and by difficulties with his eldest son.

William was the son of the unmarried Robert I, Duke of Normandy, by Robert's mistress Herleva. His illegitimate status and his youth caused some difficulties for him after he succeeded his father, as did the anarchy that plagued the first years of his rule. During his childhood and adolescence, members of the Norman aristocracy battled each other, both for control of the child duke and for their own ends. In 1047 William was able to quash a rebellion and begin to establish his authority over the duchy, a process that was not complete until about 1060.

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