This volume contains the review papers presented at the International Symposium on Solar-Terrestrial Physics held at the Tavrichesky Palace, Leningrad, U.S.S.R., 11-19 May 1970. The Symposium may be regarded as the most recent member of a series of inter national symposia - for instance, the Symposium on Solar-Terrestrial Physics, Belgrade (1966), the Joint IQSY-COSPAR Symposium on Solar-Terrestrial Physics, London (1967), and the Symposium on the Physics of the Magnetosphere, Washington (1968). Like those earlier symposia, the Leningrad Symposium was sponsored by the International Astronomical Union (IAU), the International Union of Geodesy and Geophysics (IUGG), the International Union of Radio Sciences (URSI), and the ICSU Committee on Space Research (COSPAR). These bodies are all concerned with one or another aspect of solar-terrestrial physics, and all joined in believing that the time was ripe for another comprehensive symposium on all aspects of this very active field of research.