Numerous reports of ground water pollution have caused considerable attention to be given to assessing and/or quantifying subsurface transport and fate processes. A large number of research-oriented field studies of the transport and fate of selected contaminants have been conducted or are ongoing. Laboratory-oriented microcosm studies can be used in lieu of, or as a supplement to, field studies. In this context, a microcosm can be defined as a controlled laboratory system which attempts to simulate, on a small scale, a portion of a real world subsurface environment. Pritchard and Bourquin (1983) have indicated that a microcosm study involves the establishment of a physical model or simulation of part of the ecosystem in the laboratory within definable physical and chemical boundaries under a controlled set of experimental conditions. Microcosm studies have been conducted on terrestrial, surface water, and ground water systems; the emphasis of this chapter will be on the use of such studies for examining organic contaminants in ground water and subsurface environment systems.