This chapter builds on the early work of geochemists who transformed our understanding of geothermal systems. The reader will learn about some methods geochemists apply to collect, evaluate, and interpret hydrogeochemical data from geothermal systems when addressing issues concerning geothermal exploration and development. Beginning with a discussion of how geothermal fluids acquire their chemical characteristics, the following topics are covered in roughly the chronological order in which they are encountered during geothermal project development: (1) geothermal fluid geochemical data collection, including sampling and analysis of the surface manifestations of a geothermal system, well sampling for testing and reservoir monitoring, and geofluid sampling for power plant O&M; (2) predrilling fluid chemistry during geothermal exploration, geochemical processes, and interpretation; (3) application of well test geochemistry to resource assessment and development planning; (4) geochemical aspects of resource design criteria used in gathering system and power plant design; and (5) geochemical monitoring for reservoir management and to support power plant operations and maintenance.