The TanDEM-X mission, formed by the TanDEM-X satellite in cooperation with its almost identical twin TerraSAR-X (TSX), has mainly been designed to acquire bistatic synthetic aperture radar (SAR) images of the Earth. Initiated in 2010, the primary mission objectives were to generate a global digital elevation model (DEM) to perform scientific measurements and to explore novel SAR techniques. Up to the present day, all these objectives have been completed with outstanding results. In addition, the satellites maintained a stable image quality from the start throughout their entire lifetime. The ground segment, which was tailored to the capabilities of the space segment, as well as an excellent design and manufacturing of the satellites were the key elements to achieve such a performance. In addition, the elaborate calibration concept and a careful treatment of the on-board resources enabled full operability up to the present day and allowed additional mission objectives such as the generation of a Change-DEM layer as an update to the global DEM. The stable bus and SAR payload performance as revealed by long-term system monitoring provide the basis for further extension of the TanDEM-X mission for several more years. In the following paper, these long-term system monitoring results are presented to provide an overview of the evolution of the radar system. The results and experience gained with more than a decade of TanDEM-X mission operation are a great value for both the scientific and commercial community and serve as a forerunner for the development of future SAR missions.