Abstract: The features of imaging sonar, and the background and early development of synthetic aperture onar (SAS) are reviewed. The principle, difficulties and imaging algorithms of SAS are discussed, with special emphasis on ways to achieve high resolution, the influence of underwater acoustic channels on imaging, multi sub-array techniques and corresponding imaging algorithms, stable motion platforms and motion sensors, and motion compensation and autofocusing. The latest developments and future prospects of SAS are also presented.