Population mobility is commonly identified as a key driver of the HIV epidemic, both linking geographically separate epidemics and intensifying transmission through inducing riskier sexual behaviours. However, beyond the well-known case studies of South African miners and East African truck drivers, the evidence on the links between HIV and mobility is nuanced, contradictory and inconclusive and is in part attributed to the abstract definitions of mobility used in different studies. This problematic conception of mobility, with no reference to who moves, their motivations for moving, or the characteristics of sending and receiving areas, can have a dramatic impact on how one understands the influence which this structural factor has on HIV risk in different settings. Future research on mobility and HIV transmission must incorporate an understanding of migration and mobility as dynamic processes and link different patterns and forms of mobility with location-specific sexual networks and HIV epidemiology.