In Chapter 8 we discussed the theory of Fraunhofer diffraction and interference emphasizing in particular the relevance of Fourier transforms. In this chapter we shall describe the applications of interference to measurement; this is called interferometry. Some of the most accurate dimensional measurements are made by interferometric means, particularly using waves of different types – electromagnetic, matter, acoustic etc. The variety of techniques is enormous, and we shall limit ourselves in this chapter to a discussion of several distinctly different interferometric principles, without any intention of describing the variety of instruments or methods within the classes. There are several monographs on interferometry which discuss practical aspects in greater detail, for example Tolansky (1973), Steel (1983) and Hariharan (1985).