This volume presents the proceedings of the 1990 Advanced Study Institute entitled "Fundamentals of Gas Phase Ion Chemistry" held at Mont Ste. Odile , Alsace, France, 25th June -6th July, 1990. The Institute brought together over 100 physicists, physical and organic chemists working on a wide variety of topics with gas-phase ion chemistry as the common theme. Many different viewpoints, making use of very different experimental and theoretical approaches, were brought to bear on the subject and provided a stimulating and up-to-date account of the subject. Although the Institute was built around the invited lectures, many specific points were addressed in workshops which consisted of informal discussion groups which were organised by participants during the Institute. This volume therefore contains not only chapters based on the lectures but summaries of many of the workshops which adds considerably to the diversity of information presented. This Advanced Study Institute was the fifth in a series of NATO-sponsored institutes devoted to various aspects of the physics and chemistry of gas phase ions. These meetings have been held every four years since the first, held in Biarritz in 1974, considered "Interactions between Ions and Molecules". The five volumes which comprise the proceedings of these meetings illustrate very clearly the many advances in theory and experiment which have taken place over the last 20 years.