Exploring the importance of megacities and megacity-regions as one of the defining features of the 21st century, this Handbook provides a clear and comprehensive overview of current thinking and debates from leading scholars in the field. Highlighting major current challenges and dimensions of megaurbanization, chapters form a thematic focus on governance, planning, history, and environmental and social issues, supported by case studies from every continent. Analysing vital questions for contemporary urban research, this Handbook looks at: what place megacities and megacity-regions occupy in a world of cities; how they interrogate current thinking about urban society, theory, and policy; and what role these largest of urban areas will play in shaping humanity's future. Key contributions reveal that research needs to further focus a critical and analytical lens on the particularities and distinctive issues associated with megaurbanization. A timely and essential read for urban studies, urban geography, and public policy students, the interdisciplinary nature of this Handbook provides a thorough view into the features and importance of megacities and megacity-regions. Public policy-makers and planners will also benefit from the wide-ranging case studies included.