A symposium on the theory and mechanism of diffusion in solids was held at the NASA Lewis Research Center, Cleveland, Ohio, on October 24 and 25, 1960. Diffusion in solids has been receiving a great deal of widespread attention in the past few years. The purpose of the symposium was to provide an opportunity for active workers in the field to discuss the present state of knowledge. It was attended by 87 scientists from 37 university, industrial, and government laboratories in Canada, England, Italy, and the United States. The formal presentation was limited to eleven invited papers in order to allow ample time for informal discussion. The meeting was opened by E. J. Manganiello, Associate Director of the Lewis Research Center, and an introductory address was made by Ira H. Abbott, director of the NASA Office of Advanced Research Programs, Washington, D.C.