The fourth course of the International School on Physics with Low Energy Antiprotons was held in Erice, Sicily, at the Ettore Majorana Centre for Scientific Culture from 25 to 31 January, 1990. The previous courses covered topics related to fundamental symmetries, light and heavy quark spectroscopy, and antiproton-nucleus interactions. The purpose of this school is to review theoretical and experimental aspects of low energy antiproton physics concerning the quark-gluon structure of hadrons and the dynamics of the. antiproton-nucleon interaction. Another important objective is the discussion...