Christopher Pethick graduated with a D.Phil. in 1965 from the University of Oxford, and he had a research fellowship there until 1970. During the years 1966–69 he was a postdoctoral fellow at the University of Illinois at Urbana-Champaign, where he joined the faculty in 1970, becoming Professor of Physics in 1973. Following periods spent at the Landau Institute for Theoretical Physics, Moscow and at Nordita (Nordic Institute for Theoretical Physics), Copenhagen, as a visiting scientist, he accepted a permanent position at Nordita in 1975, and divided his time for many years between Nordita and the University of Illinois. Apart from the subject of the present book, Professor Pethick's main research interests are condensed matter physics (quantum liquids, especially 3He, 4He and superconductors) and astrophysics (particularly the properties of dense matter and the interiors of neutron stars). He is also the co-author of Landau Fermi-Liquid Theory: Concepts and Applications (1991). Henrik Smith obtained his mag. scient. degree in 1966 from the University of Copenhagen and spent the next few years as a postdoctoral fellow at Cornell University and as a visiting scientist at the Institute for Theoretical Physics, Helsinki. In 1972 he joined the faculty of the University of Copenhagen where he became dr. phil. in 1977 and Professor of Physics in 1978. He has also worked as a guest scientist at the Bell Laboratories, New Jersey. Professor Smith's research field is condensed matter physics and low-temperature physics including quantum liquids and the properties of superfluid 3He, transport properties of normal and superconducting metals, and two-dimensional electron systems. His other books include Transport Phenomena (1989) and Introduction to Quantum Mechanics (1991).