Migration and population mobility has long been regarded as an important structural driver of HIV. Following initial concerns regarding the spatial spread of the disease, mobile populations are viewed to engage in higher levels of risky sexual behaviours than non-mobile groups. However, beyond the case studies of mineworkers and truck drivers, the statistical evidence is inconclusive, suggesting that the relationship between mobility and risk is not well understood. This study investigated how engaging in specific livelihoods that involve mobility influences sexual behaviour and HIV risk. A qualitative research project, including focus groups and in-depth interviews with key mobile groups, was conducted in Northern Tanzania. The findings show that the patterns and conditions of moving related to the requirements of each different economic activity influence the nature of relationships that mobile groups have whilst away, how and where local sexual networks are accessed, and the practicalities of having sex. This has further implications for condom use. Risk behaviours are also shaped by local sexual norms related to transactional sex, emphasising that the roles of mobility and gender are interrelated, overlapping, and difficult to disentangle.