Techniques for estimating drug use prevalence include direct methods (enumeration of known drug users, needs assessments, population surveys, and school surveys), indirect methods (capture-recapture methods, network analysis, and ratio estimation methods), and combined methods. Examples of the use of these various methods indicate many jurisdictions in the United Kingdom are conducting prevalence and/or behavioral studies of drug use and are planning to use the findings in formulating drug policies. In the wider context, the European Drug Monitoring Centre for Drugs and Drug Addiction has recognized the need to improve prevalence estimation throughout Europe and considerable resources are being devoted to achieving this aim. To the extent drug use is illegal, however, difficulties will always remain in determining the nature and extent of the drug problem, and policymakers will need to accept a degree of uncertainty in drug use prevalence estimates. 26 references, 5 tables, and 3 figures