The XV International Workshop on Condensed Matter Theories was held at the beautiful seaside resort of Mar del Plata, Argentina, during the first week of July, 1991. The first meeting of this workshop took place at the Instituto de Ffsica Teorica, Sao Paulo, Brazil, in 1977, as the first Panamerican Workshop on Condensed Matter Theories. Its purpose was to bring together scientists from the Western countries, to work on many different topics related to the manifold aspects of condensed matter theories. The Workshop was so successful in facilitating exchanges of ideas and techniques pertaining...