Since its appearance in the nineteen fifties Synthetic Aperture Radar (SAR) has been experiencing a continuous growth in systems, performance and applications. Thanks to its possibility of any-weather and any-time operation, SAR became rapidly a primary remote sensing system for Earth and planetary observation and today a widening of its application in more demanding fields, such as natural hazards or surveillance, is noticeable: as an example, by using airborne SAR it is possible to obtain 3-D images of a surface with centimeter resolution, detect surface variation and indicate moving targets. This article intends to give an overview of SAR operation and performance, with special emphasis on aspects related to aerospace engineering. Basic geometry to define SAR observation is introduced, illustrating how dynamics of the aerospace platform embarking SAR is fundamental to image formation. It is explained how SAR operates by exploiting the range-Doppler approach and how a noteworthy increase of azimuth resolution is achievable. Multistatic configurations are then presented with special emphasis on cross-track and along-track SAR interferometry, showing how digital elevation models and moving target identification can be obtained. The principles of SAR processing are outlined and the orders of magnitude of overall configurations and achievable performance are given. Finally, a brief introduction to future SAR systems is given, discussing potentialities of multiple antennas distributed on aerospace platforms flying in formations.