This book provides a comprehensive and critical review of the application of geochemical techniques in the evaluation of geothermal resources. It describes the processes controlling geothermal fluid chemistry, and illustrates how water and gas chemistry can be used to understand the hydrology and reservoir conditions in a geothermal system. Procedures to undertake water, gas and soil geochemical exploration surveys are also described. It addresses scientists and students as well as to engineers and industrial managers but will also be useful to environmental geologists and chemists.