This chapter considers a first group of image processing operations, those procedures applied to correct some of the defects in as-acquired images that may be present due to imperfect detectors, limitations of the optics, inadequate or nonuniform illumination, or an undesirable viewpoint. It is important to emphasize that these are corrections that are applied after the image has been digitized and stored, and therefore are unable to deliver the highest quality result that can be achieved by optimizing or correcting the acquisition process in the first place. The corrections generally should be applied before enhancement is undertaken or measurements are made.