In contrast to many cities internationally, Johannesburg, South Africa, is densifying. Densification takes varying forms across the city. These processes are driven by informal processes and through markets enabled by a loosening of planning controls. In the context of international debates over urban compaction and densification, this paper draws on four Johannesburg case studies to explore the effects that densification is having in the city, and their implications for policy debates. It shows how diverse, complex and contingent densification often is, presenting a contextually situated mix of benefits and challenges. Targeting density increases in particular areas may contribute to city spatial transformation, but a wider set of processes less directed by policy may be more important in practice. Engagement with actual dynamics and outcomes of densification and a deeper understanding of the decisions of ordinary people are critical in developing appropriate policies. Findings broadly support arguments for compaction and densification, but show that experiences are mixed, and that greater attention to managing and supporting these processes is needed.