Prevalence of problem drug use (PDU) is one of five key indicators used by the EMCDDA to provide more comparable, scientifically based estimates of the prevalence of more severe patterns of drug use that cannot be reliably measured by surveys. This information is useful for assessing treatment needs, and offers a realistic basis for estimating the social costs of drug problems, for example drug-related crime. These guidelines have been produced by the EMCDDA to improve quality and comparability of national estimates of PDU prevalence. They are based on a series of seminars and projects, organised by the EMCDDA and the IFT, which brought together experts from EU countries and the EMCDDA to discuss possible methods, data availability and target definitions. The guidelines focus on the definition of the target group and data requirements rather than on the statistical properties of the methods.