This book chapter considers the nature and properties of microcosms, the variability of the subsurface environment as it affects investigations with microcosms, microcosms as physical models, and the rational design of microcosms. It also considers the properties and appropriate use of several microcosms, including (1) two microcosms to study survival of microorganisms in ground water, (2) a microcosm to study biotransformations of concentrated industrial wastes after deep-well disposal, (3) a microcosm for the study of biotransformation of pollutants which uses internal recirculation of water to allow repeated sampling, (4) a microcosm in column configuration to study biotransformations of pollutants which are highly volatile or sorb strongly to rubber or plastic, and (5) a simple batch microcosm to study biotransformations of pollutants that are volatile.