Book excerpt: TRB Commercial Truck and Bus Safety Synthesis Program (CTBSSP) Synthesis 23: Operator Drug and Alcohol Testing Across Modes explores practices used to deter drug and alcohol use among operators within the U.S. Department of Transportation (DOT) regulated community. The report includes a brief history of the transportation workplace drug and alcohol testing program, the general approach, the reasons for testing, some of the issues that impact the validity of the tests, and an outline of the specific regulations by mode. Some alcohol and drug testing statistics are presented in the report to help provide a sense of the scope of the program and of the prevalence of illegal alcohol and drug use among safety sensitive employees. The report also highlights alternative strategies aimed at helping to deter illegal alcohol and drug use among employees.