This symposium was held at the College de France in Paris from August 31 to Sep­ tember 4, 1970. The Organizing Committee consisted of V. Bumba, R. Howard (Chairman), K. O. Kiepenheuer, R. Michard, E. N. Parker, A. B. Severny, V. E. Stepanov, and T. Takakura. The Local Organizing Committee consisted of Miss G. Drouin (Secretary), R. Michard (Chairman), J. -C. Pecker, and J. Rayrole. We are indebted to the College de France for their kind hospitality. I wish to express my gratitude to members of the Organizing Committee for advice and assistance and to R. Michard and the Local Organizing Committee, who were responsible for the smooth running of the sessions, the distribution and collection of the discussion sheets, and for a delightful Wednesday afternoon excursion to Meudon. It is a pleasure to thank J. W. Evans, V. E. Stepanov, K. O. Kiepenheuer, R. G. Giovanelli, T. G. Cowling, V. Bumba, W. C. Livingston, and J. M. Wilcox who kindly served as session chairmen. I also wish to thank Miss Judy Harstine and John M. Adkins of the Hale Observatories, for invaluable assistance in editing the proceedings. This Symposium has been supported financially by the International Astronomical Union.