With a volume of about 8–9 km3, the Flims Rockslide is the largest rockslide in Europe. Its deposits have been intensively studied in over 70 publications, but very little research on the source zone and initiation mechanisms has been conducted. This paper summarises the first detailed work investigating the kinematics and mechanics of the failure, as well as the geologic and geomorphologic controls on the rockslide, using rock mass characterization, geomorphological and tectonic mapping, and two-dimensional numerical modelling. The catastrophic Flims Rockslide required a weak marl layer, pore water pressures, and possibly seismic loading to fail. It was structurally controlled, as indicated by the scarp geometry that parallels pre-existing tectonic features. These structural features may have acted as boundaries to several compartments in the rockslide body, failure initiating at the toe and progressing upslope. The sliding zone is a composite sliding surface, probably formed by following pre-existing, bedding-parallel discontinuities and brittle fracture of intact rock bridges.