This paper aims to explore the nature of prevention by answering the questions: What is it? With whom should it be done? and How? Central to an understanding of prevention is the notion of 'community'. It is argued that the best prevention goal is a 'harm reduction' rather than targeting preventing drug 'use' or 'abuse'. It is suggested that prevention efforts are more likely to be effective if they are supported by a critical mass of the power in the community of interest, and are implemented from within the community, rather than imposed from outside it. Drawing heavily on the work of Holder (1989) and Strauss and Corbin (1990) the paper offers a preliminary model which is an attempt to provide examples of prevention activities, and their mechanisms of action, which can be applied at various levels of community from the international level, to the interaction of a small group of users at the time of using a substance. It is suggested that in the drug arena prevention has been more effective when strategies have been implemented at a number of these community levels.