This paper examines the applicability of the diffusion model to drug use. A variety of studies that employ the diffusion model to examine drug-related behaviour, including the use of alcohol, tobacco and illicit drugs, are discussed. Studies include those that focus on "natural" or spontaneous diffusion and those in which diffusion of a drug or a drug-related intervention is planned. Most studies examined support the application of the diffusion model in the study of drug use. The model is particularly valuable when new substances are introduced to a population or subgroup. The addition of economic and other forms of availability as a determinant of adoption would increase the power of the model. Recommended directions for research are outlined. The diffusion model has been used successfully in the study of drug use for several decades. More rigorous designs would strengthen this type of research and provide direction for policymakers and those involved in public health and education.