Since the first edition of this book, the field has developed rapidly, causing Nobel prizewinning author Sir Nevill Mott to rewrite this second edition extensively. It deals in an elementary way (as far as possible) with electrons in non-crystalline systems. It builds on the successful first edition by bringing up to date the theory of interactions in non-crystalline systems, introducing a revised discussion on the `minimum metallic conductivity', and addressing the relevance of disorder in the new high-temperature superconductors.