Geology, exploration, and initial developments of significant geothermal areas of the world are summarized in this report which is divided into two parts. Part 1 is a review of the geological and explorational aspects of geothermal energy development; areas of potential development in the Western United States are also discussed. The most favorable geological environment for exploration and development of geothermal steam is characterized by recent normal faulting, volcanism, and high heat flow. Successful exploration for steam consists of coordinated multidisciplinary application of geological, geophysical, and geochemical knowledge and techniques. These are reviewed. California leads in known geothermal reserves and is followed by Nevada, Oregon, and New Mexico. Specific prospective areas in these 11 Western States are described.