Phillipe Nozieres: professor of physics at the College de France, Paris, he studied at the Ecole Normale Superieure in Paris and conducted research at Princeton University. Dr. Nozieres has served as a professor at the University of Paris and at the University of Grenoble. His research is currently based at the Laue Langervin Institute in Grenoble. A member of the Academie des Sciences, he has been awarded the Wolf Prize, the Holweck Award of the French Physical Society and the Institute of Physics, and the Gold Medal of the C.N.R.S. Dr. Nozieres' work has been concerned with various facets of the many-body problem, and his work currently focuses on crystal growth and surface physics. David Pines: centre for Advanced Study professor of physics at the University of Illinois at Urbana-Champaign, he has made pioneering contributions to an understanding of many-body problems in condensed matter and nuclear physics, and to theoretical astrophysics. Editor of Perseus' Frontiers in Physics series and the American Physical Society's Reviews of Modern Physics, Dr. Pines is a member of the National Academy of Sciences and the American Philosophical Society, a Foreign Member of the Academy of Sciences of the U.S.S.R., and is a Fellow of the American Academy of Arts and Sciences and the Feenberg Memorial Medal for Contributions to Man-Body Theory in 1985, the P.A.M. Dirac Silver Medal for the Advancement of Theoretical Physics in 1984, and the Friemann Prize in Condensed Matter Physics in 1983.