IAU Symposium 40, on Planetary Atmospheres, was held at Marfa, Texas, in the Paisano Hotel, October 26-31, 1969, hosted by The University of Texas at Austin and the McDonald Observatory of the University of Texas. The Organizing Committee consisted of: Dr. Tobias Owen (Illinois Institute of Technology), Dr. Carl Sagan (Cornell University), Dr. John Hall (Lowell Observa­ tory), Dr. Arvydas Kliore (Jet Propulsion Laboratory), Academician Georgi I. Petrov (Institute of Cosmic Research, Soviet Academy of Sciences), Dr. V. I. Moroz (Stern­ berg Institute), Dr. H. C. van de Hulst (Leiden University). Local arrangements were under the care of Dr. Harlan Smith (McDonald Observa­ tory), assisted especially by Dr. Ronald Schorn (Jet Propulsion Laboratory), Mr. Curtis Laughlin (McDonald Observatory), and Miss Virginia Church (University of Texas); also by Dr. Joseph Chamberlain (Kitt Peak National Observatory), Dr. Maurice Marin (McDonald Observatory), Mr. and Mrs. J. Bergstrahl and Mrs. K. MacFarlane (McDonald Observatory), and Dr. Donald Rea (NASA Headquarters). The scientific program was divided into three parts; Venus, Mars, and the outer planets. Detailed programs for these sections, and their subsequent editing, were respectively in the hands of the undersigned. It was possible to hold the Symposium because of generous sponsorship by the Executive Committee of the International Astronomical Union, including the provi­ sion of a travel grant for assisting younger astronomers to attend, and also including the active attention and assistance of the Assistant General Secretary, Dr. C. de Jager.