Prion diseases are transmissible neurodegenerative disorders that include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle and Creutzfeldt-Jakob disease (CJD) in humans. The principal component of the infectious agent responsible for these diseases appears to be an abnormal isoform of the host-encoded prion protein (PrP), designated PrP(Sc). Prion diseases are transmissible to the same or different mammalian species by inoculation with, or dietary exposure to, infected tissues. Although scrapie in sheep has been recognized for over 200 years, it is the recent epidemic of BSE that has centred much public and scientific attention on these neurodegenerative diseases. The occurrence of variant CJD in humans and the experimental confirmation that it is caused by the same prion strain as BSE has highlighted the need for intensive study into the pathogenesis of these diseases and new diagnostic and therapeutic approaches. The existence and implications of subclinical forms of prion disease are discussed.