Expression of prion protein (PrP) genes in transgenic (Tg) mice has been an extremely effective means of studying human and animal prion diseases. Indeed, much of what we currently understand about the molecular basis of prion pathogenesis derives from such studies. Despite these advances, the emergence of a new variant of Creutzfeldt-Jakob disease (vCJD), apparently the human manifestation of bovine spongiform encephalopathy (BSE), demonstrates that our understanding of the factors controlling prion transmission is far from complete. We review studies in Tg mice that have addressed issues of prion strains and species barriers and have provided insights into mechanisms of prion propagation. The goal of future investigation will be to determine the interplay between PrP primary structure and conformation in determining prion transmission barriers and we discuss some ongoing transgenic studies designed to address these issues.