[en] The ultimate goal of controlled nuclear fusion research is to make a new energy source available to mankind, a source that will be virtually unlimited and that gives promise of being environmentally cleaner than the sources currently exploited. This goal has stimulated research in plasma physics over the past two decades, leading to significant advances in the understanding of matter in its most common state as well as to progress in the confinement and heating of plasma. An indication of this progress is that in several countries considerable effort is being devoted to design studies of fusion reactors and to the technological problems that will be encountered in realizing these reactors. This range of research, from plasma physics to fusion reactor engineering, is shown in the present three-volume publication of the Proceedings of the Fourth Conference on Plasma Physics and Controlled Nuclear Fusion Research. The Conference was sponsored by the International Atomic Energy Agency and was held in Madison, Wisconsin, USA from 17 to 23 June 1971. The enthusiastic co-operation of the University of Wisconsin and of the United States Atomic Energy Commission in the organization of the Conference is gratefully acknowledged. The Conference was attended by over 500 scientists from 24 countries and 3 international organizations, and 143 papers were presented. These papers are published here in the original language; English translations of the Russian papers will be published in a Special Supplement to the journal Nuclear Fusion. The series of conferences on Plasma Physics and Controlled Nuclear Fusion Research has become a major international forum for the presentation and discussion of results in this important and challenging field. In addition to sponsoring these conferences, the International Atomic Energy Agency supports controlled nuclear fusion research by publishing the journal Nuclear Fusion, and has recently established an International Fusion Research Council. The primary aim of this Council, which had its first meeting in conjunction with the Madison Conference, is to promote international co-operation in controlled nuclear fusion research and its application. By these activities the International Atomic Energy Agency hopes to contribute significantly to the attainment of controlled fusion power