Serodiscordant primary relationships, in which one partner is HIV-positive and the other is HIV-negative, are increasingly recognised as a key context for the transmission of HIV globally. Yet insights into the dynamics of serodiscordance remain relatively limited. I argue that to understand what makes serodiscordant couples engage in sexual practices that increase the chance of transmission, we need to examine what HIV 'risk' actually means in different cultures and contexts. A 'socially situated' approach to HIV risk moves beyond its scientific conceptualisation as an objective 'fact', revealing a diversity of perceptions and competing risks. It also reveals that couples do not necessarily perceive their mixed HIV status in terms of 'difference', a common assumption that predetermines serodiscordance and thereby obscures its many and complex enactments. I draw on examples from the social research literature to illustrate how serodiscordance is shaped in different ways by local practices, priorities, and meanings. I argue that it is within these lived contexts that perceptions and negotiations of 'risk' arise and, thus, where couples' sexual practices need to be situated and understood. Such insights are timely as HIV research and prevention grapple with emerging scientific data that challenge traditional understandings about HIV transmission risk.