Since the mid-1970s, researchers have noted the substantial increase in heroin abuse in most European countries and increasing drug abuse in southern and eastern Europe, where previously there had been little illicit drug use. Drug abuse is usually assessed using routine statistics such as arrests of drug offenders and seizures of illicit drugs, the number of addicts entering treatment, hospital admissions for drug-related illnesses, and drug-related deaths, which are available but unreliable. Special epidemiological studies, for example, case-finding studies; surveys of the general population, students, and conscripts; studies of agency populations; ethnographical studies; studies of illicit markets; and statistical projections are more reliable but also more expensive. The author advocates more systematic and centralized monitoring of routine data from both treatment and law enforcement sources; improved intra-European cooperation in such activities would enhance the value and utility of epidemiological work. Specifically, researchers need to develop common criteria and definitions, as well as data-gathering instruments, and countries need to improve the means through which knowledge and experience in drug abuse assessment is exchanged. 5 references. (Author abstract modified)