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 level, finally reached the current level of research. The sequence of lectures and the organization of the material taught were in keeping with a didactical presentation. In essence the Institute had the two-fold purpose of organizing what was known on the subject, and updating the knowledge in the field. Fif'teen series of lectures for a total of 44 hours were given. Five one-hour seminars and five twenty-minute seminars were presented. A total of 57 participants came from 40 laboratories in the following countries: Belgium, Canada, France, Germany, Ireland, Israel, Italy, Netherlands; Polatid, Romania, Switzerland, the United Kingdom, and the United States. The secretaries of the Institute were: D. Pacheco for the scientific aspects and A. La Francesca for the administrative aspects of the meeting. These proceedings report the lectures, the one-hour seminars (abstracts only) and the twenty-minute- seminars (titles only). The proceedings report also the contributions sent by Prof. K. Rebane and Dr. L. A. Rebane who, unfortunately, were not able to come.