William FitzOsbern, 1st Earl of Hereford, Lord of Breteuil, was a relative and close counsellor of William the Conqueror and one of the great magnates of early Norman England. FitzOsbern was created Earl of Hereford in 1067, one of the first peerage titles in the English peerage. He is one of the very few proven companions of William the Conqueror known to have fought at the Battle of Hastings in 1066. His chief residence was Carisbrooke Castle on the Isle of Wight, one of many castles he built in England.

William the Conqueror, sometimes called William the Bastard, was the first Norman king of England, reigning from 1066 until his death. 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.