On behalf of all those who have for the past two years devoted much time and effort in preparation for the XI. International Astronautical Congress with the aim of making it as good as possible it is my privilege to present herewith the Proceedings of the main sessions of this congress, Volume 1. In a time of tremendous scientific and technical activity and an exponentially growing accumulation of experimental data and knowledge, the dissemination of scientific results becomes increasingly important. This has a bearing not the least on such relatively new domains as space science and astronautics. Scientific presentations and discussions, and personal contacts established at international congresses and symposia play a role in this respect. But also the written word has its great importance, and, surely, every congress participant wishes to get in his hands the printed congress proceedings as soon as possible. It is with this vivid and legitimate wish in mind that the organizers of the XI. International Astronautical Congress in Stockholm have made a serious effort to present already at the open- ing of the congress at least a part of the accepted papers in a printed volume. For various reasons it has not been possible to include in this volume more than thirtysix of the seventyfive papers that will be presented at the main sessions of the congress.