As of 2008, for the first time, the majority of the world’s population is living in urban areas, many in megacities (with populations over 10 million). Megacities are not only the center of growing economies, but are also large sources of air pollutants and climate-forcing agents. Under this initiative an assessment has been written that for the first time summarizes the current knowledge around atmospheric chemistry in megacities in Africa, Asia, South America, North America, and Europe. The assessment also summarizes past and current research projects on this topic such as MEGAPOLI, CityZen, ICARTT, CalNex, MILAGRO, CareBeijing, PRIDE-PRD, and IMPACT. Finally the report will identify knowledge gaps on atmospheric chemistry in megacities. IGAC plans to provide updates to this assessment every 4 to 5 years.