The Air Transportation Systems Lab (ATS Lab) is concerned with the demand characteristics for passenger and freight movements, technological change of aircraft and their enabling infrastructure, airport capacities and flight delays, and aircraft movements. Jointly, these air transportation system components determine aviation sector fuel use and emissions of pollutants and noise. The ATS Lab collaborates with a number of research institutions, including MIT, Georgia Tech, Ecole Nationale de l’Aviation Civile and Imperial College London. Our current research is directed at the costs for reducing aircraft fuel burn and CO2 emissions. Future work examines the range of implications associated with airport capacity expansions.