This chapter focuses on ISAR signal processing techniques. As already stated in Chapter 1, ISAR is another way to face the problem of forming high resolution radar images of targets. Differently from SAR, ISAR techniques exploit the target motion to form e.m. images of the illuminated targets. Since the targets are typically non-cooperative, their motions are unknown. Then, to coherently process the targets echoes, the relative motion between the radar and the target must be estimated first by means of autofocusing techniques. This is the key point that differentiates SAR from ISAR.