The book introduces the topic of geochemical modeling of fluids in subsurface and hydrothermal systems. The intention is to serve as a textbook for graduate students in aqueous, environmental and groundwater geochemistry, despite the fact that its focus is on the special topic of geochemistry in hydrothermal systems, it also provides new insights for experienced researchers with respect to the topic of reactive transport. The overall purpose is to give the reader an understanding of the processes that control the chemical composition of waters in hydrothermal systems and to highlight the interfaces between chemistry, geothermics and hydrogeology. From the reviews: "..is a nice, compact introduction to the principles of modeling coupled fluid flow and fluid-mineral reactions in active geothermal systems, as used for heating and electricity generation." ( Christoph A. Heinrich, ECONOMIC GEOLOGY, June 2004)