Book excerpt: For the first time in human history, more than half the world’s population is urban. A fundamental aspect of this transformation has been the emergence of giant cities, or megacities, that present major new challenges. This book examines how issues of megacity development, urban form, sustainability, and unsustainability are conceived, how governance processes are influenced by these ideas, and how these processes have in turn influenced outcomes on the ground, in some cases in transformative ways. Through 15 in-depth case studies by prominent researchers from around the world, this book examines the major challenges facing megacities today. The studies are organized around a shared set of concerns and questions about issues of sustainability, land development, urban governance, and urban form. Some of the main questions addressed are: What are the most pressing issues of sustainability and urban form in each megacity? How are major issues of sustainability understood and framed by policymakers? Is urban form considered a significant component of sustainability issues in public debates and public policy? Who are the key actors framing urban sustainability challenges and shaping urban change? How is unsustainability, risk, or disaster imagined, and how are those concerns reflected in policy approaches? What has been achieved so far, and what challenges remain? The publication of this book is a step toward answering these and other crucial questions.