There is no paucity of books on high pressure. Beginning with P. W. Bridgman's The Physics of High Pressure, books of general interest include the two-volume Physics and Chemistry of High Pressure, edited by R. S. Bradley, and the series, Advances in High Pressure Research, as well as the report on the Lake George Conference in 1960. Solid state physics is well represented by Solids Under Pressure, edited by Paul and Warschauer, by Physics of Solids at High Pressure, edited by Tomizuka and Emrick, and by Properties Physiques des Solides sous Pression, edited by Bloch, as well as by chapters in Volumes 6, 13, 17, and 19 of Solid State Physics, edited by Seitz, Turnbull, and Ehrenreich. Chemistry in gases and liquids is covered in Weale's Chemical Reactions at High Pressure, and Hamann's Physico-chemical Effects of Pressure. In addition to the coverage of techniques and calibrations in the above volumes, Modern Very High Pressure Techniques, edited by Wentorf, High Pressure Methods in Solid State Research, by C. C. Bradley, The Accurate Characterization of the High Pressure Environment, edited by E. C. Lloyd, and a chapter in Volume 11 of Solid State Physics are devoted entirely to this facet of high pressure research. It is not our plan either to supersede or extend these approaches. It is our purpose here to discuss the effect of high pressure on the electronic properties of solids.