This book focuses on the socio-economic and sustainability challenges facing megacities in dealing with the dramatic population increases of informal areas and settlements (or slums), especially when coupled with the impacts and risks of climate change. The authors examine informal urban areas globally and in developing countries utilizing strategic environmental assessment (SEA) as a tool to solve the sequence of upgrading steps concerning slums and shanty towns, and also establish essential guidelines for local governments and stakeholders to create a balance and quality of life for slums dwellers, particularly in the age of the COVID-19 pandemic, through applying sustainability indicators that enhance the upgrading process. Coverage includes recent statistics and mapping of informal areas worldwide and assessment of the GIZ and Sir Norman Foster models in terms of energy demands and consequential emission of CO2 and air pollution from slums. Three models of Maspero's Triangle are also studied and assessed. The book is essential reading for a wide range of researchers, students, policymakers, governments, and professionals as well as a good source for research centers and academicians working in energy, climate change, urban environments, and sustainable urban development.