As for the first item, the number of users of positron facilities would increase eight-fold if most of the people that perform photoemission experiments and measure the presence of vacancies would like to have another and possibly better look with positrons. The volume of useful results would presumably increase accordingly. The real breakthroughs cannot be imagined, but the long stated possibilities of making a positronium Bose-Einstein condensate, observing stimulated annihilation, and making an annihilation gamma-ray laser might be sufficient for the present.

Since the 1993 School, a long way has been gone along a path through technical achievements and useful applications of positron spectroscopy, but only the first steps have been made toward what should properly be called physics with many positrons, i.e. physical situations where interactions of positrons with positrons can be observed. This decisive progress was made possible by the developments of positron storage techniques, allowing positrons bursts to be delivered on a target with high space-time density. This is a subject that draws an intense research effort, thus progress is expected soon. At the same time, new intense positron beams based on nuclear reactors and on accelerators have become available. This is a crucial advance also for experiments based on non-interacting positrons but nevertheless requiring high positron fluxes to be feasible with good statistics in a reasonable time. This book, which includes all the above aspects, can be thought to consist of two parts. The first part presents recent results or speculations regarding future experiments where the positron-positron interaction is an essential factor as well as experiments with single positrons but still requiring positron storage or intense primary sources. The latter category includes the production and the study of antihydrogen and the manipulation of positronium which is necessary for efficient antihydrogen production. The second part of the book focuses on the production of high positron fluxes and densities. Here existing intense sources are described and new possibilities are suggested. The basics and the progress envisaged with positron traps is thoroughly discussed in the final chapters.

The present editors wistfully acknowledge the fact that the present School is the child and grandchild of previous ones that were made possible by the contributions of many colleagues, especially Werner Brandt. These pages record in formal notes the distillation of all night lecture preparations, lively discussions, and thoughtful conversations as well as the fruit of many years of work by each of the participants. Our gratitude to our fellow lecturers and students and to the SIF staff who made our school such a pleasant and profitable event is beyond our capacity to say, even as we are unable to record the ambiance of Villa Monastero with its mountain lakeside scenery, midnight storms, and azure skies.

R. S. Brusa, A. Dupasquier and A. P. Mills jr.