In an urbanizing world, healthy cities are becoming an increasingly important global policy priority. The United Nations Sustainable Development Goals explicitly targets several critical urban health issues, including those linked to the built environment. But the notion of healthy cities is far from new. Its conception dates back to the mid-1980s, when scholars and practitioners sought to understand and address the links between urban living conditions and health, and the vital role played by local governments in health promotion. What has changed over time is the nature and scale of urban inequality, especially as the gravity of global urbanization has shifted away from North America and Europe toward Africa, Asia, and Latin America. This chapter examines how urban inequalities have broadened and deepened in different parts of the world. The focus is specifically on low- and middle-income countries where informal settlements, often pejoratively termed “slums,” are becoming sites of some of the deepest forms of poverty and inequality, especially in a changing climate. The chapter concludes by highlighting the challenges that urban inequality poses for health promotion in the pursuit of healthy cities, emphasizing the need for multi-sectoral and inter-disciplinary approaches based on examples, including water and sanitation, public transport, and urban violence.