Current tests can detect almost all drugs or their metabolites. The tests commonly used in urine screening programs are based on immunoassays or thin-layer chromatography. Screening tests are not definitive, however. Confirmatory tests are necessary to distinguish between positive results due to the drug's presence in the urine specimen, positive results due to cross-reactivity of the test with other substances in the urine specimen, or to testing errors. The tests all have intrinsic limitations, and errors are inevitable. Properly performed tests are especially crucial in mass screening programs, where the chance for error is greater and where even a small error rate will represent large numbers of people. Hair analysis may soon emerge as a new screening technology. Costs and cost-effectiveness are important factors to consider in deciding whether to institute a drug screening program. In addition, information on the technical capabilities and limitations of urine drug tests can be of assistance in evaluating the reasonableness of a mandatory drug testing program in the workplace. 50 footnotes and appended charts.