A number of important exploration and reservoir production questions can be answered from studies of the chemistry of geothermal fluids and reservoir rocks, and so geochemistry plays a relatively important role in geothermal exploration and development. Geochemical reconnaissance involves sampling and analyzing waters and gases from hot springs and other geothermal manifestations in the area under investigation. The data obtained are then used to help locate a geothermal system, to determine whether the geothermal system is hot-water or vapor dominated, to estimate the minimum temperature expected at depth, to predict the homogeneity of the water supply, to infer the chemical character of the waters at depth, and to determine the source of recharge water. Geochemical principles can also be applied to interpretation of chemical data from producing wells and may yield information on formation of scale in pipes or a gradual chemical change in the geothermal fluids that could indicate an impending change in production temperature