This Symposium was held at Surfer's Paradise, Queensland, Australia, from 7 to 11 September 1973. The Organizing Committee, chaired by J. P. Wild, consisted of A. Boischot, A. Bruzek, J. T. Jefferies, G. Newkirk, T. Takakura, and V. V. Zheleznyakov. We are indebted to the Local Organizing Commettee, chaired by S. F. Smerd and including R. G. Giovanelli, R. E. Loughhead, N. G. Seddon, K. V. Sheridan, and J. P. Wild, for advice in preparing this volume as well as for the smooth arrangement of the sessions. In addition, the session chairmen and reporters are to be thanked for their assistance in preparing the recorded discussions. It is a pleasure to thank Mrs R. Toevs and Mr A. Csoeke-Poeckh of High Altitude Observatory for assistance in editing these Proceedings. The financial aid for the Symposium afforded by the International Astronomical Union, the Ian Potter Foundation of Melbourne, and the Sunshine Foundation of Melbourne, as well as generous assistance of the CSIRO Divisions of Physics and Radiophysics is gratefully acknowledged.