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. This is the first volume of the final report an EMCDDA project ('Prevalence estimates of problem drug use'), which took place in 2001–02. The project had four main aims : collection of updated national estimates and methodological details from Member States; scientific assessment of quality and comparability of the estimates; assess the state of implementation of the EMCDDA key indicator and discuss gaps and obstacles; give comments on instruments and mechanisms for reporting data to the EMCDDA. Volumes 2 and 3 can be downloaded by clicking on the relevant links in the table of contents below.