TanDEM-X is a mission proposal for an innovative spaceborne radar interferometer which has been evaluated in a phase A study by a joint DLR and EADS/Astrium team. The mission concept is based on two TerraSAR-X radar satellites flying in close formation to achieve the desired interferometric baselines in a highly reconfigurable configuration. Major mission goal of TanDEM-X is the generation of a very accurate and consistent Digital Elevation Model (DEM) on a global scale. For this, appropriate orbit concepts have been introduced which allow for the generation of a global DEM according to the HRTI-3 standard within 3 years. The achievable height accuracy has been derived from a detailed performance analysis taking into account all major system and scene parameters like the finite radiometric sensitivity of the individual radar sensors, co-registration and processing errors, range and azimuth ambiguities, baseline and Doppler decorrelation, the strength and orientation of surface and vegetation scattering, quantization errors, temporal and volume decorrelation, baseline estimation errors and the chosen independent post-spacing (horizontal resolution). Furthermore, the dependency of the performance on the various operational modes and important instrument settings has been investigated and recommendations for system optimization will be given. Critical issues will be presented together with a derivation of essential requirements on both the system and mission level. Secondary mission goals of TanDEM-X are moving target indication with a distributed four aperture displaced phase centre system, the measurement of ocean currents and the detection of ice drift by along-track interferometry, high resolution SAR imaging based on a baseline induced shift of the Doppler and range spectra (super-resolution), the derivation of vegetation parameters with polarimetric SAR interferometry, large baseline bistatic SAR imaging for improved scene classification, as well as localised very high resolution DEM generation based on spotlight interferometry. The feasibility of these modes will be investigated and preliminary predictions about the achievable performance for each mode will be made.