Geothermal energy technology is reviewed in terms of its current impact and future potential as an energy source. In general, the geothermal energy resource base is large and well distributed globally. Geothermal systems have a number of positive social characteristics (they are simple, safe, and adaptable systems with modular 1-50 MW [thermal (t) or electric (e)] plants capable of providing continuous baseload, load following, or peaking capacity) and benign environmental attributes (negligible emissions of CO2, SOx, NOx, and particulates, and modest land and water use). Because these features are compatible with sustainable growth of global energy supplies in both developed and developing countries, geothermal energy is an attractive option to replace fossil and fissile fuels. In 1997, about 7,000 MWe of base-load generating capacity and over 15,000 MWt of heating capacity from high-grade geothermal resources are in commercial use worldwide. A key question is whether these levels can grow to a point where geothermal energy is more universally available and thus have a significant impact on global energy supplies in the twenty-first century. Such an achievement will require the economic development of low-grade resources. The current status of commercial and emerging technologies for electric power production and direct heat use is reviewed for the major geothermal resources including hydrothermal, geopressured, hot dry rock, and magma. Typically, high-temperature resources (>150 degrees C) provide base-load generating capacity while lower-temperature resources provide energy for geothermally assisted heat pumps and for direct use in domestic, agricultural, and aquacultural heating applications. Critical development issues relating to resource quality and distribution, drilling costs, and reservoir productivity are discussed in the context of their economic impact on production costs. Advanced drilling and improved heat mining methods are suggested as approaches to increase the worldwide use of geothermal energy by reducing field development costs. With these improvements, lower-grade resources can compete in growing global energy markets that are currently controlled by abundant and low-cost fossil fuels.