The Ionospheric Institute of the National Observatory of Athens has had two interests in recent years: the study of the ionosphere and the study of the sun. In our previous Advanced Study Institutes in 1960, 1961, and 1962, we have em­ phasized the ionosphere. For the Advanced Study Institute of 1964, however, we invited Dr. Jules Aarons of the Air Force Cambridge Research Laboratories to collaborate in preparing and directing a program of studies of the sun, the moon, the planets, and the interplanetary medium. The lectures of this Advanced Study Institute form essentially an advanced course in radio astronomy. Without being a textbook on the matter, we feel that the present book can be considered as an excellent reference for those students starting their research work in the field of solar system radio astronomy. All lecturers tried to present their subjects in a simple form based upon their exten­ sive personal experience, but without emphasizing their personal research. We must recognize that it was an excellent achievement for them to keep their text exactly at the level indicated by the Program Director, and outlined by the gen­ eral program of Advanced Study Institutes of NATO. We are deeply grateful to all the invited scientists for their outstanding contributions in lecturing on their subjects in a clear and authoritative manner. The Scientific Affairs Division of NATO, in its aid to basic research, spon­ sors various programs. Among them is the Program of Advanced Study Institutes.