An increasing abuse of drugs among young people emerged in northwestern Europe during the late 1960s and early 1970s. Since the mid-1970s, the two most important trends have been the substantial increase in heroin abuse in most countries and increasing drug abuse in southern and eastern European countries that had relatively little previous experience with illicit drug use. Drug abuse is assessed using routine statistics, which are available but unreliable, or special epidemiological studies, which are more reliable but also more expensive. There is a need for more systematic and centralized monitoring of routine data from both treatment and enforcement sources; improved co-operation in such activities between European countries would enhance the value and utility of epidemiological work. Common criteria and definitions, as well as data-gathering instruments, need to be adopted so that data on drug abuse will be more comparable. The ways and means through which countries exchange knowledge and experience gained in drug abuse assessment need to be improved.