Microbially mediated, naturally occurring transformations of subsurface and groundwater contaminants have been observed at many sites. These protective biotransformation processes, which have been called natural attenuation, assimilative capacity, or intrinsic bioremediation, are potentially useful as a component of remedial actions. If future remedial action decisions are based on cost/risk analysis, then in situations where public health is not threatened, the long-term monitoring and operational time periods, which may be expected with natural remediation methods may become acceptable. However, natural attenuation must be predictable and well understood to be accepted as a viable alternative to conventional physical and biological treatment methods. More information is needed about the environmental parameters, abiotic processes, microbial interactions, and metabolic responses that control the initiation, rate, and extent of these transformation and degradation processes in the subsurface. Research is in progress to define these factors and to develop conceptual and predictive models that describe the response of subsurface microbial communities to introduction of organic and inorganic contaminants.