Urine drug testing (UDT) has become an essential feature of pain management, as physicians seek to verify adherence to prescribed opioid regimens and to detect the use of illicit or unauthorized licit drugs. Results of urine drug tests have important consequences in regard to therapeutic decisions and the trust between physician and patient. However, reliance on UDT to confirm adherence can be problematic if the results are not interpreted correctly, and evidence suggests that many physicians lack an adequate understanding of the complexities of UDT and the factors that can affect test results. These factors include metabolic conversion between drugs, genetic variations in drug metabolism, the sensitivity and specificity of the analytical method for a particular drug or metabolite, and the effects of intentional and unintentional interferants. In this review, we focus on the technical features and limitations of analytical methods used for detecting drugs or their metabolites in urine, the statistical constructs that are pertinent to ordering UDT and interpreting test results, and the application of these concepts to the clinical monitoring of patients maintained on chronic opioid therapy.