As is apparent from the table of contents, the lectures at the Third Course of the International School of Applied Geophysics, Erice, March 27-April 4, 1980 (the first part of this volume) dealt with several applications of inversion to different geophy sical methods. For every field, the more general lectures come first, followed by those aimed at more specialized objectives. Not all topics are covered and the coverage is not uniform. The seismological section (especially the seismic reflection methods) is the most developed, and this is only partly due to the actual state of the art. Unfortunately, only abstracts are available for two of the lectures. The second part of the volume contains some short notes and contributions presented either by the lecturers themselves or by other participants. They do not necessarily deal with the process of inversion itself but with the preparation and meaning of the data to be inverted or with some original treatments of problems that were discussed in the afternoon sessions. The discussion sessions and the round table that followed the lectures were essential to the success of the Course and to an understanding ot the difterent perspectives of the various specialists. I hope that the group of very brilliant and willing geophysi cists that made the meeting so interesting will stay ~n touch, grow closer, and meet again. Close scientific cooperation among them could contribute much to the unification of geophysical science.