Microbiology, as it relates to ground-water pollution, can be divided into two distinct areas of interest: the fate and transport of pathogens, and the microbial ecology of subsurface regions and the potential interactions of microorganisms with chemical or microbial pollutants. This article discusses the general state-of-knowledge in these areas and presents some of the difficulties in studying subsurface microbial ecology. Determinations of the nature and extent of microbial activity in the subsurface, the movement and fate of pathogens, and the controlling environmental factors are essential if ground-water quality is protected.