Philip I, commonly known as Philip of Alsace, was count of Flanders from 1168 to 1191. Philip took the title of Count of Flanders already in 1157, when his parents Thierry of Flanders and Sibylla of Anjou left Flanders on crusade. After the return of his father Thierry, Philip co-ruled with him until his fathers' death in 1168. During his rule Flanders prospered economically. He took part in two crusades and died of disease in the Holy Land during the Third Crusade at the Siege of Acre.