A series of remarkable discoveries in the past three decades have led to the molecular and genetic characterization of the transmissible pathogen causing scrapie in animals and a quartet of human illnesses: kuru, Jakob-Creutzfeld disease, Gerstmann-Sträussler-Scheinker disease, and fatal familial insomnia. To distinguish this pathogen from viruses and viroids, the term 'Prion' was introduced to emphasize its proteinacous and infectious nature. This book reviews recent advances in studies of prions. Considerable evidence now indicates that prions are novel pathogens composed only of protein. The conversion of a normal cellular protein into a pathologic isoform is the fundamental event in prion replication and the pathogenesis of prion diseases.