Impairment by drugs has gained increasing attention in recent years as a possible threat to traffic safety. Research has indicated the presence of psychoactive drugs in killed and injured drivers and experimental studies have demonstrated performance impairment in subjects who have been given certain drugs. Many states and other countries have implemented laws designed to deter drugged driving. Attempts to control drugs in traffic, however, are subject to gaps in knowledge about drugs and an array of practical difficulties. They are also influenced by political concerns. In response to these issues, the Alcohol, Other Drugs, and Transportation Committee of the Transportation Research Board held a workshop to discuss the current state of knowledge of drugs in traffic. The workshop took place at the Jonsson Conference Center of the National Academy of Sciences in Woods Hole, Massachusetts, on June 20-21, 2005. This report provides an overview of the information presented and the discussions among participants as well as the background papers prepared for the workshop.