The 6th ESLAB Symposium, organised by the Space Science Department (formerly ESLAB) of the European Space Research and Technology Center, was held in Noord­ wijk from 26-29 September 1972. This year the theme was "Photon and Particle Interactions with Surfaces in Space". More than 60 scientists attended mainly from ESRO Member States and from America. The first part of the Symposium was devoted to introductory lectures and to papers on interactions with spacecraft. The second half dealt with the photon and particle interactions with celestial objects, and ended with a general discussion and presenta­ tions of areas where new developments are required. The purpose of this Symposium was to throw light on the importance of the prob­ lems which are evoked by E. A. Trendelenburg in his introductory remarks, and to sum up our present understanding of these phenomena. It is hoped that this book will prove useful to physicists and engineers who are actually involved in space ex­ periments and are concerned with interactions of these types. R. J. L. GRARD OPENING ADDRESS Gentlemen, I should like to welcome you to the 6th ESLAB Symposium. In the past we have always organised this Symposium jointly with our sister in­ stitute, ESRIN, in Frascati, but unfortunately reductions in the scientific budget have forced ESRO to terminate the activities of that laboratory. Nevertheless, we have decided to carryon the tradition, and we shall continue on our own organising this series of symposia on specialised subjects.