From 1066 until 1204 it was held by the kings of England, except for the brief rule of Robert Curthose (1087–1106), eldest son of William the Conqueror but unsuccessful claimant to the English throne; and Geoffrey Plantagenet (1144–1150), husband of Empress Matilda and father of Henry II.
In 1202, Philip II of France declared Normandy forfeit to him and seized it by force of arms in 1204. It remained disputed territory until the Treaty of Paris of 1259, when the English sovereign ceded his claim except for the Channel Islands; i.e., the Bailiwicks of Guernsey and Jersey, and their dependencies (including Sark).
In the Kingdom of France, the duchy was occasionally set apart as an apanage to be ruled by a member of the royal family. After 1469, however, it was permanently united to the royal domain, although the title was occasionally conferred as an honorific upon junior members of the royal family. The last French duke of Normandy in this sense was Louis-Charles, duke from 1785 to 1789.
The first Viking attack on the river Seine took place in 820. By 911, the area had been raided many times and there were even small Viking settlements on the lower Seine. The text of the Treaty of Saint-Clair-sur-Epte has not survived. It is only known through the historian Dudo of Saint-Quentin, who was writing a century after the event. The exact date of the treaty is unknown, but it was likely in the autumn of 911. By the agreement, Charles III, king of the West Franks, granted to the Viking leader Rollo some lands along the lower Seine that were apparently already under Danish control. Whether Rollo himself was a Dane or a Norwegian is not known. For his part, Rollo agreed to defend the territory from other Vikings and that he and his men would convert to Christianity.
The territory ceded to Rollo comprised the pagi of the Caux, Évrecin, Roumois and Talou. This was territory formerly known as the county of Rouen, and which would become Upper Normandy. A royal diploma of 918 confirms the donation of 911, using the verb adnuo ("I grant"). There is no evidence that Rollo owed any service or oath to the king for his lands, nor that there were any legal means for the king to take them back: they were granted outright. Likewise, Rollo does not seem to have been created a count or given comital authority, but later sagas refer to him as Rúðujarl (earl of Rouen).