This NIJ Research in Brief discusses trends in marijuana use among booked adult arrestees from 1987 through 1999. Results were detected through urinalyses served by the Arrestee Drug Abuse Monitoring program at 23 locations nationwide. It also explores trends within the mainstream population based on self-reports of past-month marijuana use recorded by the National Household Survey on Drug Abuse (NHSDA) and Monitoring the Future (MTF) programs and presents key findings.