The role of hydrochemistry in geothermal exploration and development is described. During the surface exploration phase hydrochemistry is particularly useful in delineating regional ground-water movement, in estimating underground temperatures, and in mapping the areal extent of geothermal systems. From data obtained through exploratory and investigation, drilling hydrochemistry yields information on the direction of underground geothermal water flow, mixing processes, boiling in the aquifer, and distribution of underground temperatures. It also contributes significantly in defining and solving scaling and corrosion problems and is relevant for environmental aspects of fluid disposal and constructional design.