This volume presents the Proceedings of International Astronomical Union Symposium No.93 on Fundamental Problems in the Theory of Stellar Evolution. It contains the texts of all the invited papers, the abstracts of the contributed papers that were read by one of the attending author(s), and edited discussions. Only one abstract is included in this volume from each author who attended, and the abstracts of papers which were read on behalf of absent author(s) are not included. Those papers, which were read but are not included in the volume, are indicated by asterisks in the table of contents. The meeting took place at the University Hall, Kyoto University, Kyoto, Japan from July 22 to 25, 1980, and was sponsored by IAU Commission 35 on Stellar Constitution and co-sponsored by the IAU Commission 42 on Close Binary Stars. Locally, the Symposium was hosted by the Research Institute for Fundamental Physics, Kyoto University with encouragement from the Astronomical Society of Japan. Financial support for the meeting was provided by the IAU, the Japan Society for the Promotion of Sciences, the Japan World Exposition Commemorative Fund, and the Yamada Science Foundation. Preparation for the Symposium and editing of the Proceedings were supported in part by Scientific Research Fund of Japanese Ministry of Education, Science and Culture (530603).