Increasing emphasis is being placed on the need for 'structural interventions' (SIs) in HIV prevention internationally. There is great variation in how the concept of SI is defined and operationalised, however, and this has potentially problematic implications for their likely success. In this paper, we clarify and elucidate what constitutes a SI with particular reference to the structured distribution of power and to the role of communities. We summarise the background to the growing emphasis being placed on the concept of SIs in HIV prevention policy, and present ethnographic case-study material from a sex worker's HIV project in Kolkata, India, to illustrate the nature of HIV vulnerability and its implications for the design and targeting of successful SIs. The paper draws attention to the dual importance of (1) attending to local complexities in the micro and macro-level structures that produce vulnerability; and (2) clarifying the meaning and role of communities within SIs.