Over half of the world’s population resides in urban areas and this number is projected to nearly double by 2050. This report provides an initial assessment of available information on air pollution and climate impacts in megacities globally. It stresses that, with the growing trend towards urbanisation, understanding the role of megacities in local to global atmospheric chemistry is critical to effectively realise the co-benefits of controlling air pollution and reducing climate change. According to the report, the scientific and engineering knowledge that has been accumulated when developed megacities dealt with their air quality problems in earlier years is a significant resource for current and future megacities. The report concludes that, unless city planners and development actors increase efforts to reduce pollution, rapid urbanisation trends will pose serious health risks to urban dwellers.