The research objectives are threefold: - First, we want to map the prevalence of poly substance use and the characteristics of poly substance users in out- and inpatient substance abuse treatment in Belgium; - Second, we will explore the extent and type of psychiatric complaints and disorders among persons following substance abuse treatment in Belgium and the prevalence and type of DSM Axis I and II-disorders in this population; - Third, we will compare the characteristics and psychiatric profile of poly substance users with that of persons who only use one substance.