From September 22 to 26 of 1997, the Twenty-First International Workshop on Condensed Matter Theories was held in Luso, a picturesque and fashionable resort in the middle of Portugal, famous, since Roman times, for its hot springs. The invited talks presented at the Workshop are recorded in the chapters of this volume. That the CMT Workshop series has reached adult status, is clearly confirmed by the strength and vigour shown and by the important position it has been occupying, on its own right, in the scientific community. The International Workshop on Condensed Matter Theories, founded in 1977, is an annual activity providing a forum for advanced theoretical research on many particle systems, with a tradition of strong involvement of participants from Developing Countries.