The traditional models of HIV transmission have envisioned sex workers as a primary ‘core group’ and their male clients as the ‘bridging population’ to female members of the public (e.g. wives, girlfriends). This project aims to reconsider these traditional HIV transmission models and assumptions in order to develop a more sophisticated model that better reflects the complexity of how the infection spreads within different populations. The limitations of traditional concepts like ‘core groups’ and ‘bridging population’ will be revealed and a new conceptual model for core groups will be designed. This renewed conceptualisation takes more explicitly into account the levels of mobility, autonomy and power of different sub-groups of sex workers.