Density is critical to cities, but how might we conceive and research its role in urban development? We argue that a conceptualization of the ‘density dialectic’ offers a productive response. Drawing on research on urban development in Tower Hamlets (London’s densest borough), we identify the tensions and contradictions of current densification approaches. A dialectical approach illuminates those tensions, examines the range of actors, processes and social, economic and environmental concerns that become enrolled, and identifies how densification operates to accommodate its changing relations and contradictions. In a context of rapid and intense urban development, we draw on interviews with planners to show how ‘gentle’ and ‘hard’ visions of density connect, conflate and collide as the borough looks to meet challenging housing targets alongside social and environmental objectives.