Imaging systems are used for transferring information from an object in the object plane to an image in the image plane. The quality of the optics in the optical system drives the quality of the final image. This quality is measured using the optical transfer function, which comprises the modulation transfer function and the phase transfer function. An optical system has limits on the range and efficiency of spatial frequencies that can be accurately transferred from the object to the image plane. The net effect is that some of the contrast is lost in the image; thus, some of the information is lost. At this point we are moving from determining only the location of the image in the optical system to also determining the effect of the optical system on the quality of the image that is being generated. It is well appreciated that a high-quality optic will focus a flat wavefront into an Airy pattern and that deviations from these ideal conditions will lead to the production of a point spread function that is broader than and less structured than the Airy pattern. Both the point spread function and the modulation transfer function provide similar information on the quality of the imaging system. In this chapter we will explore how the performance of an optical system can be measured using the modulation transfer function and discuss the meaning of these measurements.