‘Risk’ and ‘risk behaviour’ have become keywords in discourses about AIDS. Notions of risk behaviour in the field of HIV prevention and the addictions largely derive from epidemiological categorisations which have had a key role in constructing lay and scientific understandings of the ‘problems’ of AIDS and injecting drug use. This has encouraged are stricted vision which inadequately captures risk as it is understood and experienced by drug users themselves. Drawingon case examples from qualitative research with illicit drug users, this paper explores the utility of current theories of risk behaviour in understanding how risk behaviour is socially organised. Two key developments in risk behaviour theory are examined. These are situated rationality and social action theories. Findings illustrate that situated rationality theoriestend to be conceptually limited to an analysis of individual rationality which fails to capture the distribution and influence of power in negotiated actions and the habituated nature of risk behaviour. In contrast, social action theories aim to understand the interplay of social factors which give rise to individuals’ situated risk perceptions and actions. Findings highlight the important role that qualitative research plays in questioning, as well as complementing, dominant scientific constructions of risk. Future theories of risk behaviour in the field of HIV prevention, as well as other health domains, need to consider risk as a socially organised rather than individual phenomenon. The paper concludes by asserting that understanding what risk actually means to participants themselves provides the necessary data for public health interventions to create the conditions where risk reduction becomes possible.