The prion diseases are an interesting group of neurodegenerative disorders for a variety of reasons. The most obvious is their property of transmissibility, but beyond that they constitute a fascinating example of the diversity of disease expression possible from a common etiologic factor. Thought of as "strains" in animals and phenotypes in humans, these varied expressions of prion disease are most likely due to subtle conformational changes in the pathogenic form of the prion protein. These strain-like characteristics are best exemplified in the genetic varieties of human prion disease in which specific mutations are associated with specific phenotypic profiles. This review attempts to highlight the clinical and pathologic features of the prion diseases with a particular focus on the genetic determinants that define the various familial forms and that modify sporadic and iatrogenic forms of the disease.