A computer-generated hologram is a geometric pattern that can be used as a precise reference in an optical test. Computer-generated holograms can be used to make reference wavefronts that would be very difficult and expensive to make by other methods. This paper reviews the development of computer-generated holograms for optical testing. Various encoding methods are discussed. Examples are given to demonstrate some of the properties of such holograms. A review of interferometer design leads to a discussion of how the hologram functions as part of the interferometer and of the limitations to the computer-generated hologram as used in optical testing.