Cities around the world have recently started to become ‘proactive’ initiators of climate strategies containing both mitigation and adaptation elements. The experience of these first movers has been studied and documented both empirically and, to a lesser extent, theoretically, primarily for cities in the global North. This symposium addresses related knowledge gaps by exploring case studies of urban regions in the global South confronting their projected climate change challenges, showcasing the experiences of Delhi, Santiago de Chile and Bogotá. Its specific aim is to explore the urban social response to nature change, the adaptation challenges faced by cities across the world and current practices of urban adaptation. Further, the symposium seeks to understand to what extent and in what respect current conceptual frameworks — which highlight urban ecological security and vulnerability — provide a useful context/framing to assist cities in confronting their challenges and to explain their actions. This introductory article examines current knowledge of the theory and practice of urban climate response. It introduces the concepts of ecological security and vulnerability and discusses the adaptive capacity of cities and how they are starting to respond to the emerging challenges of climate change. It concludes with a synthesis of the case articles and highlights some of the findings.