The present volume contains the texts of the invited talks delivered at the Sixth International Conference on Recent Progress in Many-Body Theories held in Arad, Israel during the period November 5-10 1989. The host institute was the Physics Department at the Ben Gurion University of the Negev. Beside the invited talks there have been also two poster sessions. The general format and style of the meeting followed closely those which had preceded it and evolved from the earlier conferences in the series: Trieste 1978, Oxtapec 1981, Altenberg 1983, San Francisco 1985 and Oulu 1987. The conferences in this series are intended to cover the broad spectrum of modem physics related to the many body problem. It starts on the smallest length scales in dealing with quark gluon plasmas and ends on the largest length scales in tackling many body problems in astrophysics. In between, it is concerned with nuclear physics, solid state physics, statistical mechanics, field theory, molecular dynamics, hydrodynamics, chaos, etc. Like in the Oulu conference, special emphasis has been put on recent developments in high Tc superconductivity. Beside the half day session devoted to it we have also organized an evening round table discussion on the future of the whole subject. Furthermore, a new, and in the opinion of the editor a very refreshing element which was absent in earlier conferences was the presentation of a few talks by experimentalists.

Lectures on the Many-Body Problem is a compilation of papers delivered at the Fifth International School of Physics, held at Ravello, Italy in April 1963. The book is devoted to the techniques of many-body theory, which are used in finding solutions to difficult problems encountered in solid-state physics. The text discusses such topics as the discontinuities in the drift velocity of ions in liquid helium; density fluctuation excitations in many-particle systems; tunneling from a many-particle point of view; the mathematics of second quantization for systems of fermions; and correlation functions and macroscopic equations. Theoretical physicists will find the monograph invaluable.

Nuclear physics is undoubtedly a many-body problem. A nice introduction into the present status of this subject may be found in the comprehensive mono graph by P. Ring and P. Schuck "The Nuclear Many-Body Problem" (Springer, Berlin, Heidelberg, New York 1980). However, in view of the many challenging problems that remain to be tackled, it is sensible to consider systems with few particles as model cases. These provide the basis for solving the sophisticated many-body problem posed by intermediate and heavy nuclei. Out of the large number of existing nuclear systems, few-particle, that is few-nucleon, systems can be singled out to form a special group. This is possi ble because a comparatively small number of degrees of freedom (or dynamic variables) is required for a complete description of such systems. In these Lectures we utilize this to study few-body systems in great detail, in particular three-and four-body systems. In contrast to published monographs on the subject, we deal not just with nucleonic degrees of freedom but consider also non-nucleonic degrees of freedom. The range of approaches and methods examined exceeds the scope of other textbooks. The Lectures are organized in such a way as to guide the uninitiated reader through the essentials of solving the dynamical equations of few-body systems directly towards practical applications. Formally oriented readers might like to supplement their reading with texts such as "The Quantum Mechanical Few Body Problem" by W. GlOckle (Springer, Berlin, Heidelberg, New York 1983).

This conference series is now firmly established as one of the premier series of international meetings in the field of many-body physics. The current volume maintains the tradition of covering the entire spectrum of theoretical tools developed to tackle important and current quantum many-body problems. It aims to foster the exchange of ideas and techniques among physicists working in diverse subfields of physics, such as nuclear and sub-nuclear physics, astrophysics, atomic and molecular physics, quantum chemistry, complex systems, quantum field theory, strongly correlated electronic systems, magnetism, quantum fluids and condensed matter physics. The highlights of this book include state-of-the-art contributions to the understanding of supersolid helium, BEC-BCS crossover, fermionic BEC, quantum phase transitions, computing, simulations, as well as the latest results on the more traditional topics of liquid helium, droplets, nuclear and electronic systems. This volume demonstrates the vitality and the fundamental importance of many-body theories, techniques, and applications in understanding diverse and novel phenomena at the cutting-edge of physics. It contains most of the invited talks plus a selection of excellent poster presentations.