Law enforcement agencies in thirty-four United States and British Columbia, Canada, rely on Drug Recognition Expert (DRE) officers to apprehend the drug-impaired driver. DREs utilize a systematic and standardized twelve-step procedure to reach three determinations: 1) that the driver is impaired; 2) that the driver is impaired by drugs, rather than suffering from a medical condition that requires intervention; and 3) the category of drug(s) that is causing the impairment. The admissibility of DRE opinion testimony, usually in conjunction with corroborative toxicological analysis, hasbeen upheld in many courts throughout the U.S. The seven DRE drug categories are not based on shared chemical structures, legality, or use if any in treatment of illness or disease. Rather, the categories are based on shared patterns of signs and symptoms. The categories used by DREs are: Central Nervous System (CNS) Depressants, Inhalants, Phencyclidine, Cannabis, CNS Stimulants, Hallucinogens, and Narcotic Analgesics. The prevalence of poly-drug use complicates the DRE determinations. This paper will provide an overview of the DRE procedures, DRE drug categories, and the DRE training program.