Protection of ground water from pollution by organic chemicals can only be achieved through proper control and management of source activities which release these substances into the terrestrial subsurface. Control and management procedures must ensure that released organic pollutants are sufficiently attenuated within the subsurface environment to prevent significant impairment of ground water quality at nearby points of withdrawal or discharge, where exposure of humans, animals, and sensitive ecosystems may occur. The only rational basis for technically effective and economical source control and management procedures is definitive knowledge of the transport and fate of organic pollutants in the subsurface, since predictions of the transport and fate of specific organic chemicals in specified subsurface environments will usually be required. The most effective approaches for predicting subsurface transport and fate of organic pollutants involve the use of mathematical models based on the subsurface processes governing pollutant behavior, which include geohydraulic processes controlling the movement of fluids through porous geological formations, abiotic retarding and transformation processes such as sorption and abiotic degradation and biotransformation processes. Inadequate knowledge of sorption, abiotic transformation, and biotransformation processes in deeper subsurface regions imposes major limitations on current predictive capabilities.