One of the best-known monastic settlements of western Europe, the abbey of Mont Saint-Michel occupies the summit of a prodigiously steep island site off the coast of Normandy in northern France. The church was built between 1023 and c. 1080–85. The monastic buildings, to the north of the church, were arranged vertically as much as horizontally, reflecting the constraints of the site. They appear to have comprised three adjacent and interconnecting buildings, two of three storeys, the other of two. However, two of these three ranges were overbuilt in the early thirteenth century by an ambitious development which became known as the Merveille (c. 1212–28). This article seeks to identify the buildings that the Merveille replaced and thus the entire complex as it existed in the twelfth century. This inevitably involves a certain amount of speculation and perhaps for this reason the complex has hitherto been largely ignored, important though it is for an understanding of the abbey’s early history. The article also discusses other building projects relevant to the monks, such as the cemetery, the twelfth-century Hôtellerie and the thirteenth-century infirmary and mortuary chapel, and analyses the genesis of the Merveille. Among the findings or propositions are that the monks’ cemetery was housed in what may once have been a ducal palace; that the abbey’s cloister occupied the same position as it does today but was at a lower, mezzanine level and was smaller than the present cloister; that the chapter house and infirmary were probably adjacent to the west walk of the cloister; that the original provision for kitchen and cellar and for sleeping space was inadequate; and that the Merveille, which was the work of Abbot Raoul des Îles, was not entirely new-build as sometimes thought, but a transformation and redevelopment of buildings that already existed.