The present volume contains the texts of the invited talks delivered at the Fifth International Conference on Recent Progress in Many-Body Theories held in Oulu, Finland during the period 3-8 August 1987. The general format and style of the meeting followed closely those which had evolved from the earlier conferences in the series: Trieste 1978, Oaxtepec 1981, Altenberg 1983 and San Francisco 1985. Thus, the conferences in this series are in tended, as far as is practicable, to cover in a broad and balanced fashion both the entire spectrum of theoretical tools developed to tackle the quan tum many-body problem, and their major fields of. application. One of the major aims of the series is to foster the exchange of ideas and techniques among physicists working in such diverse areas of application of many-body theories as nucleon-nucleon interactions, nuclear physics, astronomy, atomic and molecular physics, quantum chemistry, quantum fluids and plasmas, and solid-state and condensed matter physics. A special feature of the present meeting however was that particular attention was paid in the programme to such topics of current interest in solid-state physics as high-temperature superconductors, heavy fermions, the quantum Hall effect, and disorder. A panel discussion was also organised during the conference, under the chair manship of N. W. Ashcroft, to consider the latest developments in the extreme ly rapidly growing field of high-T superconductors."

This book is the third volume in an approximately annual series which comprises the proceedings of the International Workshops on Condensed Matter Theories. The first of these meetings took place in 1977 in Sao Paulo, Brazil, and successive workshops have been held in Trieste, Italy (1978), Buenos Aires, Argentina (1979), Caracas, Venezuela (1980), Mexico City, Mexico (1981), St. Louis, USA (1982), Altenberg, Federal Republic of Germany (1983), Granada, Spain (1984), San Francisco, USA (1985), and Argonne, USA (1986). The present volume contains the proceedings of the Eleventh Workshop which took place in Qulu, Finland during the period 27 July - 1 August, 1987. The original motivation and the historical evolution of the series of Workshops have been amply described in the preface to the first volume in the present series. An important objective throughout has been to work against the ever-present trend for physics to fragment into increasingly narrow fields of specialisation, between which communication is difficult. The Workshops have traditionally sought to emphasise the unity of physics. By bringing together scientists working in many different areas of condensed matter theory, for the dual purpose of fostering collaborations between them and promoting the exchange of ideas between various disciplines, a common language has been exposed and developed. The Editor of the first volume in the series, F. B.

The second volume of Condensed Matter Theories contains the proceedings of the 10th International Workshop held at Argonne National Laboratory, Argonne, IL, U.S.A. during the week of July 21, 1986. The workshop was attended by high-energy, nuclear and condensed-matter physicists as well as materials scientists. This diverse blend of participants was in keeping with the flavor of the previous workshops. This annual series of international workshops was"started in 1977 in Sao Paulo, Brazil. Subsequent'workshops were held in Trieste (Italy), Buenos Aires (Argentina), Caracas (Venezuela), Altenberg (West Germany), Granada (Spain), and San Francisco (U.S.A.). What began as a meeting of the physicists from the Western Hemisphere has expanded in the last three years into an international conference of scientists with diverse interests and backgrounds. This diversity has promoted a healthy exchange of ideas from different branches of physics and also fruitful interactions among the participants. The present volume is a continuation of the effort started last year when the invited papers from the 9th International Workshop were published by Plenum Press. Our only trepidation in organizing a book of this kind stemmed from the diversity of the material, which did not lend itself easily to well-defined topics. Still, the articles are loosely divided into eight categories, where the papers in each category have either a common theme or the same underlying technique.