This chapter considers what we know about urban socio-spatial segregation, mainly from the extant, mainly Global North literature. It argues that socio-spatial segregation is longstanding and near universal. While acknowledging authors who favour segregation, it is largely considered to be harmful, both to residents of the most disadvantaged areas and to urban society as a whole. The chapter than explores how urban inequalities might be different in developing countries. It suggests that the extent and negative impact of socio-spatial segregation is more exaggerated even than the Global North, driven by diverse, complex, and multifaceted emergent processes of urbanisation that are shaped by situated competitions for living space and development profits. The chapter is intended to provide essential context for interpreting the empirical, city-focussed chapters that follow.