The two primary sources of data are the results of tests performed on body fluids of drivers for the presence of alcohol or other drugs and self-reported information about drug use and driving. The findings show that alcohol is found in collision- involved truck and bus drivers in quantities and frequencies that warrant concern; for example, slightly less than 20 percent of the fatally injured drivers of tractor trailers have been drinking. Self-report data also indicate that alcohol use by truck and bus drivers is significant. Although the self-reported use of other drugs is substantially lower than the reported use of alcohol, there is evidence to indicate that at least for a subset of the drivers, the use of impairing substances on the job is not uncommon. Objective data from tests on blood or urine specimens is scarce; however, what does exist is generally consistent with the self-reported data. Although it is evident that the use of alcohol and other drugs by truck or bus drivers is sufficiently frequent to pose a safety risk, the real extent and characteristics of the problem still cannot be properly gauged, owing to severe limitations in the available data. 6 figures, 10 tables, and 41 references