This chapter provides an overview of drug use in prison. It is centered on illicit drug use in Europe, where the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) has been collecting aggregated data from various sources for 15 years. It also reviews, based on published literature, data from the four other global regions. A total of 59 studies from 31 countries in the five world regions were included for analysis. Results show that the prevalence of drug use varies greatly between studies. Lifetime prevalence of any illicit drug use in prison ranges between 2% to 76% worldwide with, in most cases, cannabis being the most frequently reported substance. More recent (past-month) illicit drug use is reported by <1% to 65% of inmates, while <1% to 39% report injecting illicit drugs during their stay in prison. Alcohol use in prison is reported in a few studies only, varying between 2% to 77% while the prevalence of current tobacco smoking ranges between 4% to 90%. In general, available data are scarce and patchy, especially outside Europe, with large variations in methodology relating to sampling strategy and measurement. This heterogeneity hampers comparison and may, in part, account for the wide range of prevalence estimates. Comparable methods and measures of drug use and its consequences in prison populations are needed to facilitate international comparisons and provide the sound information needed for development and implementation of drug interventions in various prison settings across the globe.