In this work the author presents a modern study of the cathedral of Santiago de Compostela in the north-west of Spain, renowned for its Romanesque architecture and as a destination for pilgrims. The author has focussed on the two main contributors in theconstruction of the building: Archbishop Gelmírez and the Master Mason Mateo. The discussion over dating and building progression revolves around which of these two designed, built and completed the west end. Following a detailed study of the masonry, the author discloses fresh evidence which reveals more of the original Romanesque state of the building. She also examines how the Historia Compostelana (a contemporaneous account of the life and work of Gelmírez from circa 1100 to his death in 1140), and the fifth part of the Codex Calixtinus (purportedly written by Aymery Picaud in the mid-1130s), contribute to the understanding of the architecture of the cathedral-church.