This book describes in detail the main concepts of theoretical spectroscopy of transition metal and rare-earth ions. It shows how the energy levels of different electron configurations are formed and calculated for the ions in a free state and in crystals, how group theory can help in solving main spectroscopic problems, and how the modern DFT-based methods of calculations of electronic structure can be combined with the semi-empirical crystal field models. The style of presentation makes the book helpful for a wide audience ranging from graduate students to experienced researchers. Performance of optical materials crucially depends on the impurity ions intentionally introduced into the crystalline host materials. The color of these materials, their emission and absorption spectra can be understood by analyzing the relations between the electronic properties of impurity ions and host crystal structure, which constitutes the main content of this book. It describes in detail the main concepts of theoretical spectroscopy of transition metal and rare earth ions.