In this chapter a detailed development is given of the theory and practice of the plane interferometer on the basis of the simple ray treatment. Many descriptions exist in the literature from the earliest papers of Fabry and Perot (1897, 1899), valuable review articles of e.g. Meissner (1941), Kuhn (1951) and Jacquinot (1960) and texts such as Tolansky (1947), Steel (1967) and Born and Wolf (1975). In this present chapter a very deliberate attempt has been made to key the discussion to example and illustration so that it serves as a practical guide. The important parameters of light gathering, finesse, resolving power, overlap of orders, scanning aperture, etc are described and also considerations due to the different types of light source. The use of the interferometer with photoelectric and photographic recording is outlined, and also some aspects of the analysis of spectra, the effect of plate imperfections and deconvolution. However detailed discussion of the design, construction and operation of plane interferometers is deferred to the following chapter.