DRUID (Driving under the Influence of Drugs, Alcohol and Medicines) aimed to combat the problem of driving under the influence of psychoactive substances by providing a solid scientific base for European policy makers. It brought together experienced organisations in Europe to assemble a coordinated set of data resources and measures. DRUID is an integrated European research project which consisted of different sub-projects (Work Packages) that were aimed at different topics such as the prevalence and risk of psychoactive substances, enforcement, classification of medicines, rehabilitation of offenders and withdrawal of driving licenses (www.druid-project.eu). The main objective of WP2 of DRUID was to assess the situation in Europe regarding the prevalence and risk of the use of illicit drugs, alcohol and psychoactive medicinal drugs by drivers. The prevalence of drug driving was estimated by means of roadside surveys and the prevalence of drugs in injury accidents was estimated by means of hospital surveys of seriously injured and/or killed drivers. Accident risk estimates for drug driving were assessed by relating the prevalence of drugs among the general driving population to the prevalence among seriously injured and/or killed drivers, by relating medication records to accident data and by relating substance use among accidentinvolved drivers to accident culpability.