Currently, there is no single, satisfactory method for estimating the prevalence of problematic drug use at a national level in Europe; therefore, a work group designed to improve the comparability of national estimates of problem drug use was established by the European Monitoring Centre of Drug and Drug Addiction (EMCDDA). Guidelines on methodological issues, data requirements, and limitations for local and national estimation techniques are accessible on the EMCDDA Web site. Various estimation methods were used in this study. The multiplier method used treatment data, police data, mortality data, and HIV/AIDS data. The capture-recapture method drew two samples of opiate users. If the samples are independent random selections from the population of opiate users, the proportion of opiate users reidentified in the second sample provide a good estimate for the ratio of opiate users in the first sample to the total population. The multivariate indicator method transferred information about drug use from a population in which it was observed to a target population in which it could not be observed. The limitations of the aforementioned methods of estimation are discussed in this paper. In the study, the target populations varied according to data selection. Estimates for six partially overlapping types of drug use could be identified: problem opiate use, problem opiate or cocaine use, problem amphetamine or opiate use, problem drug use, current injecting, and life-time injecting. The rates of injectors ranged from 2.6 in Germany to 4.8 in Luxembourg. Rates in Austria, Denmark, Finland, Portugal, and the United Kingdom were within this range. For problem opiate use, a group of countries with a high prevalence were identified (average rates exceeded six cases); these countries were Italy, Luxembourg, Spain, and the United Kingdom. Another group had lower prevalence (average rates near three cases); these countries were Austria, Germany, Ireland, and the Netherlands. Rates obtained for France (problem opiate or cocaine use), Finland, and Sweden (problem amphetamine or opiate use) were not directly comparable and were between these rates. Based on the results of this study, the authors advise that cross-national comparisons should be made with caution, and estimated target populations may vary greatly between countries. For estimating various forms of problem drug use at a national level, a multi-method approach is recommended. 1 table and 66 references