Each of them is based on large unselected patient populations drawn from treatment facilities and relates to the abuse of alcohol, medical drugs, and illegal or "street" drugs. These studies show that polydrug abuse is a common but varied phenomenon in different contexts. It poses special problems for therapy and social policy. Techniques of differentiating specific types of "polytoxicomania" are outlined. Three different clinical methods of classifying polydrug abuse are described in this chapter. These are the mental hospital sample, the Substance Abuse Warning System (SAWS) sample, and the Documentation System of the Special Hospitals for the Treatment of Substance Abuse (DOSY). In the mental hospital sample, four groups of polydrug abuse, besides three types of monodrug abuse for alcohol, medical drugs, and illegal drugs, were available for the classification of persons with substance abuse problems. The SAWS sample includes, as do the other two studies, data on alcohol, as modern studies should; it identifies 12 alternatives, three for monodrug and nine for polydrug abuse patterns. The DOSY sample uses the same criteria as the SAWS sample, but offers an additional differentiation into simple abuse and dependence for the different groups. The use of the classification system used by SAWS is recommended for all general purposes as well as for international comparison. 8 tables, 1 figure, and 15 references