The concept of prevention was first developed in the field of public health and epidemiology. Since the 1960s, it has been applied increasingly to illicit drug use, thus standing alongside law and treatment as a major form of social control over drugs. While a strict epidemiologic, or "contagion," model of drug use is held by some, we find this model to be of limited utility for discussing the broad range of efforts commonly defined in the field as preventive. An economic, or supply-demand, model is proposed instead. Under this model, prevention is defined as the attempt to reduce the demand for drugs. Four strategies for reducing demand are discussed: (1) coercion, or the threat of formal punitive sanctions; (2) persuasion, or education in the harmful consequences of drug use; (3) correction, or the eradication of the presumed causes of drug use; and (4) substitution, or the provision of alternatives to drug use. The limitations of each strategy are discussed, and because of the prevalence of recreational patterns of moderate drug use, it is concluded that the prevention of all illicit drug use is not an achievable goal.