The experimental transmissions of spongiform encephalopathies, neurodegenerative diseases found in humans and some animal species, allowed the important discovery of a host-encoded prion protein closely associated, if not identical, to the transmissible agent. Transmissions in mice addressed several questions regarding the understanding of the 'species barrier' that limits, or even prevents, the transmission between different species, and regarding the resistance to these diseases. The genetic control of the disease by the host could be studied in mouse models and showed the important role of the host prion gene, but several other genetic factors involved in these diseases remain to be discovered. Finally, the analysis of the features of these diseases in mice has been crucial to characterize the infectious agents and their biological properties, although the precise mechanisms underlying their apparent diversity largely remain to be elucidated.