A new Transmissible Sub-acute Spongiform Encephalopathy (TSSE) appeared in 1986 in the bovine species, mainly concerning Great Britain where more than 100,000 cases have now been identified. This review describes the features of the disease that showed it belonged to this particular group of diseases. The currently available hypothesis about the origin of the disease, in Great Britain as well as in other countries where it has also been recognized, are presented. Lastly, the potential risks for the future, in the bovine species but also for other animal species or man, are discussed, in light of the known experimental data.