William the Conqueror becomes the Duke of Normandy, reigning until 1087.

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, his hold on Normandy was secure. In 1066, following the death of Edward the Confessor, William invaded England, leading a Franco-Norman army 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.

In the Middle Ages, the duke of Normandy was the ruler of the Duchy of Normandy in north-western France. The duchy arose out of a grant of land to the Viking leader Rollo by the French king Charles the Simple in 911. In 924 and again in 933, Normandy was expanded by royal grant. Rollo's male-line descendants continued to rule it until 1135, and cognatic descendants ruled it until 1204. In 1202 the French king Philip II declared Normandy a forfeited fief and by 1204 his army had conquered it. It remained a French royal province thereafter, still called the Duchy of Normandy, but only occasionally granted to a duke of the royal house as an appanage.