Subacute transmissible spongiform encephalopathies (STSE) represent a cause of presenile dementias, found in man (Creutzfeldt-Jakob disease, Gerstmann-Straussler syndrome, Kuru), in sheep and goats (scrapie), in cattle (bovine spongiform encephalopathy) and in various mammals. It seems that the frequency of the human spongiform encephalopathy (HSE) is more frequent than thought up to now and an alimentary origin is not to be discarded. Many discussions are held concerning the nature of the transmissible agent: virus, virino, prion? Isolation of an abnormal protein, the isoform PrPSc resulting from an unknown alteration of a protein PrPc encoded in man by the chromosome 20 is presently the basis for seeking the cause of transmissible dementias. Molecular genetic studies of the gene PrP have demonstrated the existence of many mutations: are they the actual cause of the STSE or only a favouring factor? The existence of familial sporadic or infectious forms of HSE might be explained by alterations of the conformation of PrPc into PrPSc, able to induce this structure modification to last when thereafter the synthesis of the normal protein is achieved. The "chaperone" protein model can be questioned in this respect. For the moment the research work is developed into characterization of humoral or genetic markers able to detect the predisposed subjects in high-risk families. HSE are also an interesting model for other types of dementias, especially the Alzheimer disease which is still the preferred target of research in this field.