Aerosol emissions impact the Earth’s climate directly by scattering and absorbing solar and thermal radiation and indirectly by modifying cloud properties. Mounting evidence suggests that aerosols, both natural and anthropogenic, play a key role in many atmospheric processes. They act as precipitation forming agents, modify the radiative and thermal balance of the atmosphere, and alter the development of convection. Representation of the aerosol indirect effect requires the explicit parameterization of aerosol-cloud interactions. Furthermore, the modeling of the microphysical evolution of clouds must account explicitly for processes of ice nucleation and cloud droplet activation. This seminar details the implementation of a new cloud microphysics scheme that includes aerosol-cloud interactions within the NASA GEOS-5 model. Compared to the operational version of GEOS-5 the implementation of the new microphysics leads to a more realistic representation of cloud properties, in better agreement with in situ and satellite data. Using the new microphysics the aerosol indirect effect is analyzed in terms of sensitivities of different cloud and radiation quantities to aerosol abundance. The effect of ice nuclei on cloud formation is also discussed, with emphasis on the representation of ice nucleation in atmospheric models and its improvement using ultra-high resolution simulations.