``Optical Properties of Mixed Crystals" is concerned with the description of optical processes in substitutionally disordered semiconductors and insulators which can be basically described through their elementary excitations. Two of the chapters relate to the phonon response including the effect of side bands on electron transitions. Two relate to electronic spectra, one on photoelectron spectroscopy and the other on excitons. A further chapter deals with magnons in magnetic crystals and a final chapter is related to fluctuations and band edge effects. Each chapter deals with a specific class of excitation, but the book makes it clear that the fundamental structure of the excitation spectra, including band formation, band tailing and localisation is common to every type of excitation. The volume shows how some basic concepts and ideas can be widely applied to bring coherence and understanding to a diverse area of solid state physics.