The Treaty of Saint-Clair-sur-Epte was signed in the autumn of 911 between Charles the Simple and Rollo, the leader of the Vikings, for the purpose of settling the Normans in Neustria and to protect Charles' kingdom from any new invasion from the "northmen". No written records survive concerning the creation of the Duchy of Normandy.

In addition, Rollo was to be baptised and marry Gisele, the illegitimate daughter of Charles, and become the vassal of the king. This last point was complicated because Rollo refused to kneel in front of the king and kiss his foot. A compromise was reached, whereby Rollo took the foot of the king but did not kneel, taking it so high that the king lost his balance and fell.