The 6th course of the futernational School of Solid State Physics was held in Erice, Sicily, at the Ettore Majorana Centre for Scientific Culture, 19-29 June, 1995. The course was organized as a NATO Advanced Study Institute and received generous support from NATO's Nanoscale Science Program. This volume is based on the lectures presented during the course. fudispensable for the planning of the summer school was the support of the Director of the Ettore Majorana Centre, Professor A. Zichichi. We wish to express our sincere appreciation to the center staff, Dr. Maria Zaini, Dr. Alberto Gabriele, Dr. Pinola Savalli and Dr. Jerry Pilarski for their expert assistance in all organizational matters. A special word of thanks must go to the director of the International School of Solid State Physics, Professor Giorgio Benedek, not only for his valuable advice in the planning stage, but also for his active participation in the program itself. I would like to thank my coworkers, Stefan Frank, Nikola Malinowski, Renee Stotz, Frank Tast and Kristin Wirth for their valuable assistance in preparing these proceedings of the meeting. The success of a school is, in the last analysis, determined by the inter est and commitment of the lecturers and participants. I am very grateful to the lecturers for their carefully prepared formal presentations, to the par ticipants for their contributions to the spontaneous evening "workshops," and to all for their inexhaustible enthusiasm."