Guigo I also known as Guigues du Chastel, Guigo de Castro and Guigo of Saint-Romain, was a Carthusian monk and the 5th prior of Grande Chartreuse monastery in the 12th century. He was born in 1083 near the Chateau of Saint-Romain, and entered the Grande Chartreuse in 1106. He ruled the community until his death in 1136. He was a man of considerable learning, was known for his eloquence and great memory. He was a close friend of St. Bernard of Clairvaux and of Peter the Venerable, both of whom wrote of Guigo's sanctity.