CityZen (megaCITY Zoom for the ENvironment) is a three year European Commission funded project focused on the effects of megacities and emission hot spots on their local, regional and global environment, regarding both air pollution and climate change. With 16 partners from 10 countries on 3 continents the project has addressed the complex interactions of megacities within the air quality and climate change context using state-of-the-art models, in-situ measurements and satellite observations. The project has used a “hot-spot” approach looking in an integrated manner across diverse megacities in western Europe, the Eastern Mediterranean and China. This short article gives an overview of the project and outlines some of the main findings. The integrated application of observations and models has demonstrated the regional impact of megacities and articulated some of the forward looking challenges. Overall, the CityZen project has found that despite vastly different meteorology, pollutant transport, and atmospheric chemistry conditions resulting in strong regional characteristics affecting pollutant levels in the different focus regions, similar future air quality and climate change challenges will be faced across many megacities. ...