Our world is becoming more and more urban. Already, about 50% of the population lives in cities posing new challenges for sustainable development. What does sustainability mean in the context of living in a city? This book provides guidelines for sustainability assessment of urban systems to be used by experts and policy makers alike. We integrate theory and methods around sustainability assessment with concepts of systems science. We present and discuss already employed concepts of sustainability assessment and new ways of conceptualizing sustainability assessment issues in urban settings around the world. The book highlights the need to find models and solutions in order to design the sustainable cities of the future in light of the complexity of urban social life. It further presents different aspects of urban sustainability from energy to housing, to mobility and health covering social, economic and environmental aspects.