This chapter gives a brief introduction to synthetic aperture radar (SAR) to help readers gaining some basics before jumping into the advances in SAR ocean remote sensing covered in this book. The concept of how to synthesize a long aperture in order to achieve a fine-resolution SAR image is looked at first. A physical model from a system point of view is described, followed by a flow of object- data-image domains both data acquisition and image science deserve further studies. Specially, integrative study of multi-physics modeling should be put on a higher priority.