Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders affecting both humans and animals. TSEs can be of genetic, sporadic or infectious origin. The infectious agent associated with TSEs, termed prion, appears to consist of a single protein, an abnormal conformer (PrPSc) of a natural host protein (PrPC), which propagates by converting host PrPC into a replica of itself. One of the characteristics of prions is their ability to infect some species and not others. This phenomenon is known as transmission barrier. Interestingly, prions occur in the form of different strains that show distinct biological and physicochemical properties, even though they are encoded by PrP with the same amino acid sequence, albeit in presumably different conformations. In general, the transmission barrier is expressed by an incomplete attack rate and long incubation times (time from the animal inoculation until the onset of the clinical signs) which become shorter after serial inoculation passages. Compelling evidence indicates that the transmission barriers are closely related to differences in PrP amino acid sequences between the donor and recipients of infection, as well as the prion strain conformation. Unfortunately, the molecular basis of the transmission barrier phenomenon and its relationship to prion strain conformations is currently unknown and we cannot predict the degree of a species barrier simply by comparing the prion proteins from two species.We have conducted a series of experiments using the Protein Misfolding Cyclic Amplification (PMCA) technique that mimics in vitro some of the fundamental steps involved in prion replication in vivo, albeit with accelerated kinetics. The in vitro generated prions possess key prion features, i.e., they are infectious in vivo and maintain their strain specificity. We have used PMCA to efficiently replicate a variety of prion strains from, among others, mice, hamsters, bank voles, deer, cattle, sheep, and humans. The correlation between in vivo data and our in vitro results suggest that PMCA is a valuable tool for assessing the strength of the transmission barriers between diverse species and for different prion strains; we are using the method to determine which amino acids in the PrPC sequence contribute to the strength of the transmission barrier. These studies are proving very useful in evaluating the potential risks to humans and animals, of not only established prion strains, but also new (atypical) strains. For example, while classical sheep scrapie is unable to cross the human transmission barrier in vitro, bovine spongiform encephalopathy (BSE) propagated in sheep does so efficiently. In addition, we have also generated prions that are infectious to species hitherto considered to be resistant to prion disease.