While rare in humans, the prion diseases have become an area of intense clinical and scientific interest. The recognition that variant Creutzfeldt-Jakob disease is caused by the same prion strain as bovine spongiform encephalopathy in cattle has dramatically highlighted the need for a precise understanding of the molecular biology of human prion diseases. Detailed clinical, pathological and molecular data from a large number of human prion disease cases have shown that distinct abnormal isoforms of prion protein are associated with prion protein gene polymorphism and neuropathological phenotypes. A molecular classification of human prion diseases seems achievable through characterisation of structural differences of the infectious agent itself.