This text presents a complete account of electron theory in both periodic and nonperiodic metallic systems. The author's accessible approach allows for comparisons with experimental results as much as possible. The book starts with the basics of one-electron band theory and progresses to cover high Tc superconductors and quasicrystals. The relationship between theory and potential applications is also emphasized. The material assumes some knowledge of elementary quantum mechanics as well as the principles of classical mechanics and electromagnetism. Numerous exercises and an extensive list of references and numerical data are key features.