this book deals with the mutual interaction and energy transfer between electronic defect states of f centers and defect ions (e.g. rare earth elements) and neighbouring molecular defects (oh-, cn-) in alkali halides. these model defect systems exhibit a very intriguing and ambiguous character because they behave like isolated defects in one instance but like supermolecules in others. the text gives an overview of the topic and discusses possible applications. it covers a wide range of spectroscopic methods applied to the systems and reviews several approaches for their theoretical interpretation. it includes more than 100 illustrations, figures and tables as well as many previously unpublished results. this book provides a valuable resource for further studies into this and related fields.