Scrapie, a naturally occurring neurodegenerative disease of sheep and sometimes goats, is a prototypic disease for the whole group of the subacute spongiform virus encephalopathies. Kuru was the first human disease of this type to be discovered in 1957 by Gadjusek and Zigas, and its discovery opened the whole field in the human biomedical sciences by the very realization of the fact that viruses may induce disease months or even decades after infections, and that these slow virus diseases are more compatible with classical degenerations of the nervous system than with inflammatory disorders of the brain. More than a quarter of a century since discovery of Kuru, and more than a half century following the first transmission of scrapie, the very nature of the infectious virus remains unknown. This comprehensive review covers all aspects of slow unconventional virus infections known today. It includes numerous historical data, biochemistry and molecular biology of the prion protein and its gene, the role of genetics and mutations within PrP gene, spreading and targeting of the virus, biochemistry and neurochemistry of the alterations of different neurotransmitter system and neuropathology. More than 1000 references are listed and critically analyzed; the reader can find references to all experiments and laboratory findings which has ever been done in this field. Furthermore, the book offers different views on basic problems, as for example, the nature of the scrapie agent.