The Twelfth International Conference on the Physics of Semiconductors was held at Stuttgart, Federal Republic of Germany, from July 15 to 19, 1974. The Conference was sponsored by the International Union of Pure and Applied Physics and we wish to thank this Organization as well as all the other Institutions and Companies listed on the preceding page for their substantial support. About 700 scientists from 31 countries came to Stuttgart in order to attend the Conference. Following the example of the previous Conference at Warsaw, a distinction was made between plenary invited and invited papers presented at parallel sessions. Altogether, the Proceedings contains 27 invited and 206 contributed papers. The members of the International Program Committee had the difficult task of making a selection from 550 abstracts. Their work was essential for the success of the Conference and it is gratefully acknow­ ledged. The number of pages alloted to each paper had to be limited in order to keep the Proceedings within reasonable size. In general, the papers in the Proceedings have been arranged according to the Conference Program except for the following changes: Each plenary invited paper was associated with an appropriate session, the sessions were put into a slightly different order and, in a few cases, new titles were introduced.