In this chapter the theoretical aspects of ellipsometry and their applications in optics of layered systems are presented. The basic formulae of the theory of ellipsometric measurements are introduced. For this purpose the Jones and Stokes--Mueller matrix formalisms are used. By using these formalisms the individual types of ellipsometry and the most utilized ellipsometric techniques are briefly described. Furthermore, the matrix formalisms enabling us to derive the formulae for the optical quantities of optically isotropic and anisotropic layered systems are described as well. Applications of the matrix formalisms in practice are illustrated by means of three examples.