TanDEM-X is an innovative spaceborne radar interferometer designed to generate a consistent global digital elevation model (DEM) with unprecedented accuracy, meeting or surpassing the HRTI-3 specification. The mission involves two TerraSAR-X-like satellites flying in close formation, enabling the collection of bistatic data with short along-track baselines. The key elements include an innovative phase synchronization link, a novel satellite formation flying concept, and the use of new interferometric modes for system verification and DEM calibration. The paper provides a detailed overview of the mission concept, including the operational modes (bistatic, monostatic, alternating bistatic, and simultaneous transmit), the interferometric performance analysis, and an optimized data acquisition plan. The interferometric performance is analyzed in detail, considering the peculiarities of bistatic operation, and an optimized DEM data acquisition plan is derived. The paper also discusses the relative phase referencing and time referencing for accurate phase calibration and synchronization. The HELIX satellite formation, which combines an out-of-plane orbital displacement with a radial separation, is selected for the operational DEM generation, allowing for a stable height of ambiguity and flexible baseline selection. The paper concludes with a discussion of the interferometric performance in bistatic stripmap mode, including coherence estimation and the impact of quantization errors.TanDEM-X is an innovative spaceborne radar interferometer designed to generate a consistent global digital elevation model (DEM) with unprecedented accuracy, meeting or surpassing the HRTI-3 specification. The mission involves two TerraSAR-X-like satellites flying in close formation, enabling the collection of bistatic data with short along-track baselines. The key elements include an innovative phase synchronization link, a novel satellite formation flying concept, and the use of new interferometric modes for system verification and DEM calibration. The paper provides a detailed overview of the mission concept, including the operational modes (bistatic, monostatic, alternating bistatic, and simultaneous transmit), the interferometric performance analysis, and an optimized data acquisition plan. The interferometric performance is analyzed in detail, considering the peculiarities of bistatic operation, and an optimized DEM data acquisition plan is derived. The paper also discusses the relative phase referencing and time referencing for accurate phase calibration and synchronization. The HELIX satellite formation, which combines an out-of-plane orbital displacement with a radial separation, is selected for the operational DEM generation, allowing for a stable height of ambiguity and flexible baseline selection. The paper concludes with a discussion of the interferometric performance in bistatic stripmap mode, including coherence estimation and the impact of quantization errors.