This book presents theoretical treatments on various electronic and atomic processes in non-metallic materials from a unified point of view. It starts with the basic properties of semiconductors, treating the system as a macroscopic association of electrons and ions. In their ground state, fruitful results are derived, such as the band theory for electrons in a periodic lattice and a useful concept of "hole." The electron-lattice interaction is then introduced as a dynamical response of condensed matter when it is electronically excited. With the aid of proper configuration coordinate diagrams, various phenomena are precisely examined, including carrier scattering, polaron formation, lattice relaxation, Stokes shift and phonon side band in optical spectrum, intrinsic and extrinsic self-trapping, and structural changes. The book provides readers a deep understanding of the physics underlying these phenomena and excellent insight to develop their further research. Graduate students who have finished the basic study on solid-state physics and quantum mechanics and research scientists and engineers in materials science and engineering will benefit immensely from it.