This paper reviews the literature on health in the informal settlements (and “slums”) that now house a substantial proportion of the urban population in Africa, Asia and Latin America. Although this highlights some important gaps in research, available studies do suggest that urban health inequalities usually begin at birth, are reproduced over a lifetime (often reinforced by undernutrition), and may be recreated through vulnerabilities to climate change and a “double burden” of communicable and non-communicable diseases.