Most of the world's population now live in cities, but despite wide agreement on the core values of sustainable societies, municipalities are so busy solving current problems, they don't have the time or resources to plan effective action for sustainability. The Key to Sustainable Cities uses the principles of system dynamics to demonstrate how today's problems were yesterday's solutions. The book points to a new approach to city planning that builds on assets as a starting point for cities to develop healthy social, governance, economic, and environmental systems. Gwendolyn Hallsmith has worked to build sustainable communities for over twenty years as a municipal manager, a regional planning director, and with the Institute for Sustainable Communities. She lives in Marshfield, Vermont.