The interest of social scientists in complexity theory has developed rapidly in recent years. Here, I consider briefly the primary characteristics of complexity theory, with particular emphasis given to relations and networks, non-linearity, emergence, and hybrids. I assess the ‘added value’ compared with other, existing perspectives that emphasise relationality and connectedness. I also consider the philosophical underpinnings of complexity theory and its reliance on metaphor. As a vehicle for moving away from reductionist accounts, complexity theory potentially has much to say to those interested in research on health inequalities, spatial diffusion, emerging and resurgent infections, and risk. These and other applications in health geography that have invoked complexity theory are examined in the paper. Finally, I consider some of the missing elements in complexity theory and argue that while it is refreshing to see a fruitful line of theoretical debate in health geography, we need good empirical work to illuminate it.