A survey of U.S. laboratories involved in providing analytical support to the Drug Evaluation and Classification Program identified the following drugs and drug classes as being the most frequently encountered drugs in DUID cases: marijuana, benzodiazepines, cocaine, prescription and illicit opiates, muscle relaxants, amphetamines, CNS depressants, and sleep aids used as hypnotics. This paper lists drugs within these classes that should be tested for when DUID is suspected. Labs that receive specimens for testing in DUID cases should be able to routinely detect all the drugs listed. Additional drugs specific to a laboratory's demographic area may need to be included in the list. Recommendations are offered for appropriate cutoffs for screening, and confirmations in both blood and urine are provided for some of the most important analytes in DUID investigations. These thresholds were established to reflect the performance of both commercially available screening technology and confirmatory techniques, both of which are routinely used in forensic toxicology laboratories. These recommended cutoffs are based on analytical methodology and good laboratory practice rather than pharmacology or the probability of impairment. The recommended procedure for handling suspected DUID cases is that the officer making the traffic stop systematically collect and document symptoms of drug use and impairment. This requires training officers in the symptoms of drug use and impairment and techniques of accident reconstruction. Witness statements pertinent to drug impairment should also be documented. Toxicological analysis of a biological specimen is the final step in the process. 4 tables and 6 references