These proceedings report the lectures and seminars presented at the NATO Advanced Study Institute on "Optical Properties of Ions in Solids," held at Erice, Italy, June 6-21, 1974. The Institute was the first activity of the International School of Atomic and Molecular Spectroscopy of the "Ettore Majorana" Centre for Scientific Culture. The Institute consisted of a series of lectures on optical properties of ions in solids that, starting at a fundamental...