The aim of this introductory chapter is to cover some fundamental concepts on ISAR and to highlight some characteristics that make these systems unique, especially related to SAR. Although SAR and ISAR share the same underlining concept of forming a synthetic aperture, they are substantially different in the way they process the radar received signal and generate a focused image of a target. Moreover, some basic concepts such as Doppler effect and image resolution have been introduced along with that of motion-induced radar imaging. Detailed signal modeling and image formation algorithms have been left out since they deserve focused attention, and they will be treated in the next few chapters.