The spongiform encephalopathies are unusual in several respects. Firstly, they are transmissible, and in some cases inheritable. Secondly, variants of these disorders occur in many species and can be transmitted by consumption of infected material; this has led to concern as to the potential risk from eating contaminated animal products. Thirdly, increasing evidence suggests that a 'prion' protein is central to their aetiology and pathogenesis, and that no nucleic acid is involved in the infective process. The role of the prion gene and its protein is outlined and proposed as the basis for an improved classification of the transmissible dementias.