Radiation plays a crucial role in atmospheric physics. It redistributes energy, and thus controls the vertical structure of the atmosphere. The balance between incoming and outgoing radiation controls our climate. Even to understand the climate of other planets, radiation is critical. Furthermore, radiation can be used for remote sensing, by ground based meteorological instruments, such as radars, and also from satellites. These data are indispensible for operational weather forecasting and climate research. We play a leading role in developing radiative transfer models, in particular the Atmospheric Radiative Transfer Simulator ARTS.