One of the key concerns associated with the design and usage of septic tank systems is the potential for inadvertently polluting ground water. This concern is increased when considerating systems serving multiple housing units. This chapter begins with the identification of constituents of potential concern in the effluents from septic tank systems. Mechanisms of ground water contamination from septic tank systems are addressed, including the migration of pollutants through soil and ground water systems. The transport and fate of biological contaminants are described, with information included on both bacteria and viruses. The transport and fate of inorganic chemicals are also described, with emphasis given to phosphates and nitrogen compounds as well as chlorides, metals, and other inorganic contaminants. Brief information is included on the transport and fate of organic contaminants; the brief coverage is due to the lack of extensive information in the published literature. Some information on pollution control measures is presented. Ground water monitoring for septic tank systems is also addressed. Finally, a special section is included on the handling of septage.