Drug use creates a significant amount of harm in modern societies. From an evolutionary perspective, the pervasive use of drugs and the ongoing risk of drug addiction can be explained in terms of the action of drugs on evolved motivational-emotional systems. Addiction arises through interaction of these evolutionarily ancient systems, designed to promote the pursuit of natural rewards, and contemporary environments where purified and potent forms of drugs are readily available. This evolutionary analysis is extended to account for developmental patterns in problem drug use, and to explain the existence of behavioural addictions, such as problem gambling. The paper concludes by considering some of the clinical and public policy implications of the evolutionary perspective presented.