The Sellafield area lies over the transitional zone between the western margin of the Lake District Massif, of Lower Palaeozoic metamorphic and igneous rocks, and the East Irish Sea Basin of younger sedimentary rocks. This paper sets out the general geological framework, and includes aspects such as the structural evolution, sedimentary history, rock properties and fault and fracture characteristics. From this framework, hydrogeological units are identified and linked to hydrogeological parameters as the basis for general hydrogeological models of the Sellafield area. Site specific models of groundwater flow through and around the potential location of an underground repository for radioactive waste are being developed on the basis of detailed characterization of the rock mass. In particular, geological and geophysical methods are applied to improve understanding of groundwater flow and to develop hydrogeological models integrated with the local geological framework.