Solid-State Theory - An Introduction" is a textbook for graduate students of physics and material sciences. Whilst covering the traditional topics of established older textbooks, it also takes up new developments in theoretical concepts and materials that are connected with such breakthroughs as the quantum-Hall effects, the high-T superconductors, and the low-dimensional systems realized in solids. Thus, besides providing the fundamental concepts to describe the physics of the electrons and ions comprising the solid, including their interactions, the book provides a link to the experimental facts and gives the reader an excellent insight into current research fields. A compilation of review problems makes the book especially valuable to both students and teachers. The new edition offers an additional chapter on the interaction of light and matter, plus end-of-chapter problems and solutions.