The fourth course of the International School of Physics of Exotic Atoms took place at the "Ettore Majorana" Center for Scien tific Culture, Erice, from March 31 to April 6, 1984. As tradition, exotic atoms have been a tool for studying elec tromagnetic, weak and strong interactions at low energies. We felt it appropriate to have a full course devoted to a discussion of the information to be gained on the fundamental interactions from the study of low energy systems. In this kind of physics, which is characterized experimentally by very intense particle sources and very sensitive apparatuses, one can search for rare events and can perform precise measurements. Sensitive tests of the predictions of current theories of electro weak and strong interactions can thus be achieved. The course was attended by 54 participants from 23 institutes in 9 countries. The morning lectures reviewed the achievements of the field in the last few years and the afternoon seminars dealt with new results and projects.