This volume is devoted to the topic of "Monolayer and Sub­ monolayer Helium Films," which was the subject of a symposium held at Stevens Institute of Technology, June 7th and 8th, 1973. All the papers in this volume were presented at this symposium. The symposium was sponsored by Stevens Institute of Technology with support from the National Science Foundation and the Office of Naval Research, and had as Organizing Committee: Professor D. F. Brewer (University of Sussex), Professor J. G. Dash (University of Washington), Professor J. G. 'Daunt (Stevens Institute of Technology), Doctor V. J. Emery (Brookhaven National Laboratory), Professor E. Lerner (Stevens Institute of Technology), Doctor F. J. Milford (Battelle Memorial Institute), Doctor A. D. Novaco (Brookhaven National Laboratory), Professor F. Pollock (Stevens Institute of Technology), and Professor W. A. Steele (Pennsylvania State University). The symposium was largely devoted to papers on the topic of thin (unsaturated) helium films at low temperatures, many of which were of a review character. In addition some papers of a more general character were included in order to review some powerful techniques used for investigating surfaces at higher temperatures. The Editors wish to thank all the authors of these papers for their enthusiastic cooperation in the preparation of this volume.