The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) has been collecting aggregated data on illicit drug use among European prison populations for over a decade. Additional studies were identified in a literature search. Together, these sources yielded 53 studies reporting data during the period 2000 to 2008. Analysis of these studies reveals that the available data are scarce and patchy, with large variations in methodology. This diversity hampers comparison, and may, in part, account for the wide range of prevalence estimates for drug use and drug injecting, both prior to imprisonment and while in prison. This article concludes that a common instrument to measure drug use and its consequences in European prison populations is needed. (Published Abstract)