The world became mainly urban in 2007. It is thus timely to review the state of knowledge about urban health and the current priorities for research and action. This article considers both health determinants and outcomes in low-income urban areas of developing countries. The need to study urban health in a multi-level and multi-sectoral way is highlighted and priorities for research are identified. Interventions such as the Healthy Cities project are considered and obstacles to the effective implementation of urban health programmes are discussed. Concepts such as the double burden of ill health and the urban penalty are re-visited. Finally, a call for a shift from 'vulnerability' to 'resilience' is presented.