TerraSAR-X is a high-resolution X-band synthetic aperture radar (SAR) satellite due for launch in 2006. The sensor has a spatial resolution down to 1 m and operates in Stripmap, Spotlight, and ScanSAR modes with selectable or dual polarization. It can image on the left or right side of the subsatellite track, which is achieved by rolling the satellite. There are also experimental modes for wide bandwidth, providing even higher resolution, and for full polarization and along-track interferometry (ATI), the latter two being achieved by splitting the receive antenna into two halves. Owing to this high degree of flexibility of the instrument and the tight performance requirements, the calibration of the sensor is a major challenge, and new concepts are needed to keep the costs affordable. For this purpose a novel internal calibration concept was developed, the so-called PN-gating method. In using this method, individual transmit and receive modules can be characterized under the most realistic conditions. Furthermore, cost-effect concepts for external calibration are mandatory because of the large number of modes and possible antenna patterns. The characterization of the antenna is based on a precise antenna pattern model, which provides reference patterns required for the relative radiometric correction of the SAR data. The paper describes the calibration measures planned for the TerraSAR-X instrument and discusses their implementation. The concept is applicable to other advanced SAR sensors.