The aim of this volume is to provide advanced predoctoral students and young postdoctoral physicists with an opportunity to study the concepts of tunneling phenomena in solids and the theoretical and experimental techniques for their investigation. The contributions are primarily tutorial in nature, covering theoretical and experimental aspects of electron tunnel ing in semiconductors, metals, and superconductors, and atomic tunneling in solids. The work is based upon the lectures delivered at the Advanced Study Institute on Tunneling Phenomena in Solids, held at the Danish A. E. C. Research Establishment, Riso, Denmark, June 19-30, 1967. Sponsored by the Danish Atomic Energy Commission, the Nordic Institute for Theoretical Physics (NORDITA), and the Science Affairs Division of NATO, with the cooperation of the University of Copenhagen, the Technical University of Denmark, Chalmers Institute of Technology, and the University of Penn sylvania, the lectures were presented by a distinguished panel of scientists who have made major contributions in the field. The relatively large number of lecturers was, in part, made possible by the close coordination of the Advanced Study Institute with the Second International Conference on Electron Tunneling in Solids, which was held at Riso on June 29, 30 and July 1, 1967, under the sponsorship of the U. S. Army Research Office Durham. We are indebted to I. Giaever, E. O. Kane, J. Rowell, and J. R. Schrieffer for advice and assistance in planning the lecture program of the Institute.