Spongiform encephalopathies include seven neurodegenerative diseases: three in man (Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease and kuru) and four in animals (scrapie, mink encephalopathy, bovine spongiform encephalopathy and chronic wasting disease in deer and elks). They are all transmissible to a variety of species, and man-to-man propagation of the diseases in the form of iatrogenic transmission has been well-documented. The infectious agent is highly unusual and the pathogenesis of infection remains controversial. The term prion was introduced to describe the proteinaceous infectious agent. Purification of this agent yielded a unique sialoglycoprotein, associated with the neuronal cell membrane, which is all or part of the infectious agent. Molecular genetics revealed variations in the prion protein; these are linked to or associated with the inherited forms of spongiform encephalopathies: familial Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker disease. The histological triad of spongiform change, neuronal loss and astrocytosis dominate the histological picture of spongiform encephalopathies. A recent case which did not develop any of the histological hallmarks of disease, but did have genetic abnormalities typical of the disease, indicates that the true incidence of Creutzfeldt-Jakob disease may be considerably higher than previously accepted, and a combination of molecular screening and immunohistochemistry for prion protein should complement traditional neuropathology to establish the diagnosis. The descriptive term of spongiform encephalopathy may now have to be abandoned in favour of prion disease.