This volume contains the lecture notes of ten courses given at the XIV Latin American School of Physics (XIV LASP) which took place in Caracas, Venezuela, from the 10th to the 28th of July 1972. The LASP is held each year in a different Latin American country. Its purpose is to bring together young Latin American physicists at the doctorate level to attend lectures given by well known scientists. The participants are also invited to give seminars on their research work. The topics of the courses given this year were chosen according to the existent fields of interest in Latin America. Two of...