viii The growing use of NTD silicon outside the U. S. A. motivated an interest in having the next NTD conference in Europe. Therefore, the Third International Conference on Neutron Transmutation-Doped Silicon was organized by Jens Guldberg and held in Copenhagen, Denmark on August 27-29, 1980. The papers presented at this conference reviewed the developments which occurred during the t'A'O years since the previous conference and included papers on irradiation technology, radiation-induced defects, characteriza­ tion of NTD silicon, and the use of NTD silicon for device applications. The proceedings of this conference were edited by Jens Guldberg and published by Plenum Press in 1981. Interest in, and commercial use of, NTD silicon continued to grow after the Third NTD Conference, and research into neutron trans­ mutation doping of nonsilicon semiconductors had begun to accel­ erate. The Fourth International Transmutation Doping Conference reported in this volume includes invited papers summarizing the present and anticipated future of NTD silicon, the processing and characterization of NTD silicon, and the use of NTD silicon in semiconductor power devices. In addition, four papers were presented on NTD of nonsilicon semiconductors, five papers on irradiation technology, three papers on practical utilization of NTD silicon, four papers on the characterization of NTD silicon, and five papers on neutron damage and annealing. These papers indi­ cate that irradiation technology for NTD silicon and its use by the power-device industry are approaching maturity.