During 1972 and 1973 the U. S. Geological Survey (U.S.G.S.) conducted detailed geological, geophysical, hydrological, and geochemical investigations in Long Valley, California, as part of a new geothermal research program. The goal of these investigations was to understand a typical hot water geothermal system, thus providing a basis for extrapolation to other hot water areas and for regional exploration and assessment of geothermal resources. Although the U.S.G.S. investigations have thoroughly characterized the surface expression and geophysical signatures of the Long Valley geothermal system, our understanding of the geothermal system at depth is incomplete. The available data allow us to make only a crude estimate of 350--700 MW cent for the electric power generation potential. Refinement of this estimate must await exploration of the area by deep drill holes.