HCV infection is a major public health problem in Australia, with an estimated 11,000 new infections per year occurring predominantly (>95%) among IDUs. The basic epidemiology of HCV among IDUs in Australia is well understood, but despite many findings being common across groups, rates of HCV transmission vary widely in different groups of IDUs. Individual behavioural risk does not explain these differences, and it may be that the explanation lies with the nature of IDU social networks. This research employs complementary social research/ethnographic and molecular epidemiological approaches to characterise fully the determinants of HCV transmission among IDUs. This improved knowledge of social network dynamics and their influence, and contact between risk networks, will be immensely valuable in combating HCV transmission.