Chemically active species in the atmosphere (e.g., aerosols, ozone, methane) play important roles in climate and air quality. Atmospheric concentrations of these species respond relatively rapidly to changes in emissions, providing opportunities for rapid response mitigation options. Because of the large spatial and temporal variability of these species, their global distributions must be calculated with a model, as must their influence on climate. Models are also needed for future projections of these species and their interaction with climate. Understanding what controls the distribution of these species, their role in climate change, how they will change with climate, and the coupling between climate and air quality is therefore critical.