The cities of the 21st century are the largest sites of human settlement today and are increasingly acting as critical nexus points of social, economic, ecological and technological change. This is especially evident in the developing world city context, where growth is most rapid and where future sustainability challenges will be most severe - all this in the light of growing inequalities, poverty and the pervasiveness of slums and informality. This publication presents a rationale for socially inclusive urban transitions to sustainable growth and draws on a range of case studies and theoretical and analytical considerations to establish the basis of the argument. It outlines practical steps that cities can take towards resource efficiency, including: the integration social measures into urban development; governance approaches that engage local communities and civil society groups; financial incentives and subsidies for green technologies; technology transfer; and innovation.