The Proceedings contain the papers presented at the Twelfth Symposium on Optical Materials for High Power Lasers held at the National Bureau of Standards in Boulder, Colorado, on September 30 - October 1, 1980. The Symposium was jointly sponsored by the National Bureau of Standards, the American Society for Testing and Materials, the Office of Naval Research, the Defense Advanced Research Projects Agency, the Department of Energy, and the Air Force Office of Scientific Research. The Symposium was attended by over 150 scientists from the United States, the United Kingdom, Japan, France, and West Germany. It was divided into sessions devoted to the following topics: Materials and Measurements, Mirrors and Surfaces, Thin Films, and finally Fundamental Mechanisms. The Symposium Co-Chairmen were Dr. Harold E. Bennett of the Naval Weapons Center, Dr. Alexander J. Glass of the Lawrence Livermore Laboratories, Dr. Arthur H. Guenther of the Air Force Weapons Laboratory, and Dr. Brian E. Newnam of the Los Alamos Scientific Laboratory. They also served as editors of this report.