The modulation transfer function (MTF) is used to specify the image quality achieved by an imaging system. It is particularly useful in analyzing situations in which several independent subsystems are combined. This tutorial provides the background information necessary for the application of MTF techniques to performance specification, estimation, and characterization of optical and electro-optical systems. Readers are introduced to the basic assumptions of linear systems theory, including the concept of spatial frequency, and the relationship between impulse response, resolution, MTF, OTF, PTF and CTF. Practical measurement and testing issues, coupled with several concrete examples, are presented.