The third volume of the series entitled "Symposia on Theoretical Physics" comprises the lectures delivered at the First Matscience Summer School on Theoretical Physics held in Bangalore for three weeks from August 24 to September 13, 1964. The academic program of the summer school consisted mainly of several invited lectures by both foreign and Indian scientists. Among the participants were the following: Professor R. Oehme, University of Chicago (United States); Professor K. Symanzik, New York University (United States); Professor E. R. Caianiello, Director, Institute for Theoretical Physics, Naples (Italy); Professor W. Brenig, Max Planck Institute (West Germany); Professor F. Calogero, University of Rome (Italy); Dr. A. Fujii, School of Science and Technology, Sophia University (Japan); Dr. J. Lukierski, University of Wroclaw (Poland). All were visiting scientists at Matscience. It was a very fortunate circumstance that this summer school was held immediately after the International Conference on High-Energy Physics at Dubna, U .S.S.R. Among the various topics discussed at the Dubna Conference, of particular interest is the reported violation of CP invariance and, hence, the violation also of time reversal ( T) invariance in some weak interactions as well as successful demonstra­ tion of SU 3 symmetry of elementary particle interactions. These were summarized by Professor Ramakrishnan who had attended the Dubna Conference.