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 from 1035 onward. By 1060, following a long struggle to establish his throne, his hold on Normandy was secure.

In 1066, following the death of Edward the Confessor, William invaded England, leading an army of Normans to victory over the Anglo-Saxon forces of Harold Godwinson at the Battle of Hastings, and suppressed subsequent English revolts in what has become known as the Norman Conquest. 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, Robert Curthose.

William was the son of the unmarried Duke Robert I of Normandy and his mistress Herleva. His illegitimate status and his youth caused some difficulties for him after he succeeded his father, as did the anarchy which 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.

His marriage in the 1050s to Matilda of Flanders provided him with a powerful ally in the neighbouring county of Flanders. By the time of his marriage, William was able to arrange the appointment of his supporters as bishops and abbots in the Norman church.

His consolidation of power allowed him to expand his horizons, and he secured control of the neighbouring county of Maine by 1062. .