This online review is the complete version of the 2004 Critical Review on Megacities and Atmospheric Pollution of the Air and Waste Management Association (A&WMA). In this review, the driving forces behind the formation and growth of megacities and their consequences are described. The nature of megacities, their air quality problems, and the associated science are briefly addressed. Impacts of emissions and the ambient concentration of pollutants in megacities on the health of their populations, visibility (urban and regional haze), ecosystems (including acid and fixed nitrogen deposition, photochemical oxidant damage, and photosynthetically active radiation), climate change, and global pollutant transport are evaluated. The air quality assessment tools available for large urban centers are discussed. The review looks at nine case cities representing the relatively clean urban areas in industrialized nations to highly polluted cities in the developing world: 1) Los Angeles, USA; 2) Mexico City, Mexico; 3) Toronto, Canada; 4) Delhi, India; 5) Beijing, China; 6) Santiago, Chile; 7) Sao Paulo, Brazil; 8) Bogota, Colombia; and 9) Cairo, Egypt.