Transmissible spongiform encephalopathies (TSE) are a group of diseases which have received a lot of attention in recent years. The interest on these diseases has been stimulated by the appearance of bovine spongiform encephalopathy (BSE) and the new variant of Creutzfeldt-Jakob disease (nvCJD); the latter is likely to be acquired by ingesting contaminated beef. Until now 109 cases of nvCJD have been reported, most of them occurring in the United Kingdom. Some experts think that this is the beginning of a nvCJD pandemic. Deep knowledge of the mechanisms of transmission of TSE is needed to prevent the emergence of a TSE pandemic in humans. We address various aspects of TSE and discuss prevention methods of TSE in ruminants and humans.