The protein-only hypothesis posits that the infectious agent causing transmissible spongiform encephalopathies consists of protein and lacks any informational nucleic acids. This agent, termed prion by Stanley Prusiner, is thought to consist partly of PrP(Sc), a conformational isoform of a normal cellular protein termed PrP(C). Scientists and lay persons have been fascinated by the prion concept, and it has been subjected to passionate critique and intense experimental scrutiny. As a result, PrP(C) and its isoforms rank among the most intensively studied proteins encoded by the mammalian genome. Despite all this research, both the physiological function of PrP(C) and the molecular pathways leading to neurodegeneration in prion disease remain unknown. Here we review the salient traits of those diseases ascribed to improper behavior of the prion protein and highlight how the physiological functions of PrP(C) may help explain the toxic phenotypes observed in prion disease.