Nearly one in four households in the United States depend on onsite wastewater treatment systems (OWTS) that treat wastewater from an individual home or business and dispose of it at that location.  Domestic wastewater contains a mixture of inorganic and organic contaminants, including nitrogen, phosphorus, pathogens (bacteria, protozoa, virus), and a new class of compounds referred to as emerging contaminants that include pharmaceuticals/household chemicals/natural hormones.  The conventional OWTS consist of a septic tank and a drain field (also known as a leach field, soil infiltration or absorption system, or soil treatment unit).  More advanced technologies that include treatment of raw wastewater using aerobic and/or anaerobic treatments have gained popularity in recent years as concerns have grown about protecting sensitive environments.  Relatively simple in design, OWTS can be a cost-effective and reliable means of removing contaminants from household wastewater before discharging it into the environment.  The goal of this presentation is provide an overview of the OWTS including types and functions of OWTS, wastewater flow, and typical chemical and microbiological characteristics of wastewater.  Subsequent presentations in the symposia will take a closer look at various contaminants present in OWTS, specifically the fate and transport of nitrogen, pathogens (bacteria, protozoa, viruses), and emerging contaminants.