The first exploration for geothermal energy in the U.S. apparently took place during the early 1920s in California and at Yellowstone Park in Wyoming. Despite this early start, by the 1950s, commercially developable geothermal steam had been identified only at one location, The Geysers in California. In the past 10 years, interest in developing geothermal energy has advanced rapidly. Many tens of wildcat wells have been drilled in the search for new energy sources. Many more tens of geothermal prospects are being evaluated. This evaluation of new prospects is beginning to have an impact on the science of geophysical exploration. Some geothermal reservoirs can be located merely by drilling close to thermal manifestations such as geysers and fumiroles. In many other cases, leakage of water in hot springs is far removed from the reservoir in which the hot fluids are present, and it is conceivable that many geothermal systems may exist without any surface manifestation. Development of these more or less hidden geothermal systems will require the services of geologists, geochemists, and geophysicists.