Large urban areas, commonly referred to as megacities, generally consume a huge amount of energy due to the high population density along with concentrated economic activities, and the resulting environmental burden—including air pollution—is an ongoing scientific and public issue. As several common factors and drivers cause air pollution problems in urban areas, it is widely thought that the air quality trends in different cities exhibit similarities in certain aspects correlated with urban development activity and air pollution mitigation practices. However, unique characteristics in the emission, chemistry, and dispersion of air pollutants that are specific to each urban area also exist, which often reflect differing approaches to air pollution control policy. Based on this reasoning, several comprehensive studies in big megacities, mostly in North America and Europe, have been implemented extensively over the last two decades. However, the air quality is often much poorer in the megacities of developing countries, and extensive research is still needed to tackle the urgent goals of understanding the increasing complexity of air quality and identifying appropriate mitigation measures. This MAPS (Megacity Air Pollution Studies) special issue highlights recent scientific findings from megacity air-quality research in many parts of the world. It focuses primarily on the mechanisms and drivers that result in high ozone and aerosol events in megacities but also addresses the latest advances in precursor emissions inventories, chemical transformation assessments, and forecasting models with sets of field observations, including remote sensing applications.