The use of field-theoretic techniques and methods has witnessed an impressive growth in recent years. A wealth of new and important results have emerged in condensed-matter physics, mainly in connection with low-dimensional systems such as quantum Hall systems, quantum wires and quantum dots and spin chains and/or ladders. This book gathers together in a coherent and up-to-date fashion the contributions of some of the most prominent scientists in the field. It will be of great benefit to both senior scientists and young researchers active in condensed-matter theory.