Geochemistry has contributed significantly to the understanding of ground-water systems over the last 50 years. Historic advances include development of the hydrochemical facies concept, application of equilibrium thermodynamic theory, investigation of oxidation-reduction processes and dating of ground waters.  Several other hydrochemical concepts, tools and techniques have also elucidated mechanisms of flow and transport in ground-water systems, and have unlocked an archive of paleoenvironmental information.  Today, a wide variety of hydrochemical and isotopic techniques are used to: 1) delineate solute sources and sinks, 2) interpret the origin and mode of ground-water recharge, 3) refine estimates of timescales of recharge and ground-water flow, 4) decipher ground-water reactive processes, 5) provide paleohydrological information, and 6) help calibrate ground-water flow models.  The information includes concentrations of minor and major elements, dissolved gas concentrations (N₂, noble gases, chlorofluorocarbons, sulfur hexafluoride), stable isotopes (¹⁸O, ²H, ¹³C, ³⁴S, ¹⁵N, and isotopes of Ca, Sr, B, Li, Fe and Cl), and radioactive isotopes and/or their decay products (¹⁴C, ³H, ³H/³He, ³⁹Ar, ³²Si, ³⁶Cl, ⁸¹Kr, ⁴He).  Progress needs to be made in obtaining representative samples.  Improvements also are needed in the interpretation of basic data, and in the construction and interpretation of numerical models.  The best approach will ensure an optimized and iterative process between field data collection and analysis, interpretation, and the application of forward, inverse and/or statistical modeling tools.  Future research advances in ground-water geochemistry are likely to follow from microbiological investigations, the characterization of natural organics, isotopic fingerprinting, and from improvements in the knowledge of reaction kinetics and coupled processes. The investigation of ground-water systems that are complex or difficult to observe may help in this endeavor.  A thermodynamic perspective is offered that could potentially facilitate understanding of the multiple physical, chemical, and biological processes affecting ground-water systems.