Substance abuse is a significant problem in the United States, with cocaine, marijuana, alcohol and heroin as the most commonly abused drugs. This article focuses on urine drug testing to evaluate potential drug abuse or overdose in the emergent care setting using qualitative immunoassays. Discussion is included regarding the principles of how to validate qualitative immunoassays; how to decide on appropriate specimen type, test menu and cutoff; the limitations of immunoassays; how to communicate test results to clinicians; and use of urine drug testing at point of care.