Bjorn Stevens is a director at the Max-Planck-Institute for Meteorology and an honorary professor at the University of Hamburg. His research aims to understand how water in the atmosphere, especially in the form of clouds, shapes climate. He has played a leading role in developing and executing major international field programmes, modelling studies, and scientific assessments related to clouds and climate. He served as a lead-author of Chapter 7, 'Cloud and Aerosols' for the 5th Assessment Report of the Intergovernmental Panel on Climate Change and co-leads (with S. Bony) the World Climate Research Programme (WCRP) Grand Science Challenge on Clouds, Circulation and Climate Sensitivity.