This paper traces the technical development of geothermal plants from the very beginning, focusing on the efficiency of the geothermal-to-electrical energy conversion. The plants at Larderello (Italy) and Wairakei (New Zealand) are examined in some detail. Included are discussions and analyses of littleknown plants in Africa and on the Italian island of Ischia that pioneered the development of geothermal energy conversion systems beyond the dry steam plants in Tuscany. Presented are important milestones in geothermal power plant development that opened the way to worldwide expansion of the use of geothermal resources for electricity generation. It is shown that while there has been a slight trend toward higher efficiency over the years, the improvement is less than has occurred in conventional fossil-fueled power plants. The efficiency gains result mainly from more elaborate conversion systems whose extra capital costs may not be justified in all cases. Nevertheless, the data show that geothermal power plants can be designed and built to achieve high efficiencies, comparable to or even greater than conventional plants when appropriate economic conditions and incentives exist. (C) 2014 Elsevier Ltd. All rights reserved.