This graduate textbook designed for students in physics, chemistry and materials science provides a modern treatment of the theory of solids dealing with the physics of electron and phonon states in crystals and how they determine the structure and properties of solids. The first part of the book deals with electrons and atoms in a crystal, and the second part extends the discussion to defects in crystals and to structures without crystalline symmetry. There are numerous exercises throughout and appendices to provide the necessary background.