Against a backdrop of unprecedented levels of urbanization, 21st century cities across the globe share concerns for the challenges they face. This Companion provides a framework for understanding the city as a critical building block for a more sustainable future within broader subnational, national, and continental contexts, and ultimately, within a global systems context. It discusses the sustainable strategies being devised, as well as the methods and tools for achieving them. Examples of social, economic, political and environmental sustainable policy strategies are presented, and the extent to which they actually increase sustainability is analyzed.