The purposes of drug testing by the criminal justice system are to screen for persons who have recently ingested a drug, to identify chronic drug users, to monitor and deter drug use, and to estimate national and local drug-use trends among offenders. Urinalysis is the primary metabolic indicator of drug use. The cheaper, more rapid urine tests currently available tend to be highly sensitive; because of their cross-reactivity problems, however, they should be confirmed by a second test using a different methodology. Urine testing is thus suitable for screening or monitoring large numbers of persons. Structural indicators of drug use have only recently been developed for possible use by the criminal justice system. The method receiving the most attention is the analysis of hair specimens by an experimental technique by which the drug is extracted from the hair and analyzed using radioimmunoassay tests. Urine testing has been used in pretrial testing, postadjudication testing, the testing of juvenile detainees, and the Drug Use Forecasting project. Some general findings from such testing are reviewed in this essay. A review of ethical and legal issues in drug testing concerns privacy, protections against illegal searches and seizures, and due process rights. Future research possibilities are discussed. 7 tables, 2 figures, and 155 references