This paper introduces the processing techniques of Sidescan Sonar. These techniques are not used in general image Processing. Generally, sidescan images have geometric distortions and intensity distortions. The former are discrepancies between the relative location of features and their true location, and the latter are deviations from the ideal linear relation between image intensity and backscattering strength. If the raw data include the navigation information, the geometric distortions can be corrected from it, otherwise, the attitude of vehicle can be inferred from the sonar image.