Module Description: Lab testing in the assessment of substance use disorders may involve many different substrates, though urine drug testing (UDT) dominates clinical practice. This module is meant to provide a short guide to effective use of lab testing in treatment of the patient with a substance use disorder, which to a surprising degree is often misused or misunderstood. The module describes the clinical role of lab testing to guide treatment and not to be punitive. To properly use and interpret UDT, it reviews common false positive and false negatives that might occur in the immunologic-based first step in UDT. To understand confirmatory gas chromatography/mass spectrometry, it goes on to describe the metabolism of common opioids and benzodiazepines, because the detected confirmed moiety often is just a downstream metabolite of several possible parent compounds to which the patient was exposed. The module concludes with a discussion of the difference between UDT for clinical purposes versus forensic ones. Educational Objectives: