Tandem-L is a proposal for an innovative interferometric radar mission to monitor the Earth system and its intricate dynamics. Important mission objectives are global inventories of forest height and above-ground biomass, largescale measurements of Earth surface deformations due to plate tectonics, erosion and anthropogenic activities, observations of glacier movements and 3-D structure changes in land and sea ice, and the monitoring of ocean surface currents. The mission concept is based on co-flying two fully-polarimetric L-band SAR satellites in a close formation. Tandem-L employs new techniques and advanced technologies to achieve its ambitious mission goals. The feasibility of a joint DLR/JAXA mission is currently being investigated in the scope of a pre-phase A study.