Under the Clean Water Act, the states adopt water quality standards (WQSs) for every water body within their borders. These standards include a designated use such as fishing, swimming, boating, wildlife habitat, agriculture, and drinking water supply and prescribe numeric criteria to protect that beneficial use. Depending on the standard adopted for a particular water body, controls may be needed to reduce the pollutant levels discharged to it. The law focuses primarily on surface water. Groundwater is included indirectly through regulations that cover the interaction between surface water and groundwater and through requirements in the act for groundwater protection strategies. This chapter provides an overview of the Clean Water Act through a discussion of its early history, details of the act, Water Quality Standards, Water Quality Criteria, Total Maximum Daily Load (TMDL) assessments, the National Pollutant Discharge Elimination System (NPDES) permitting system, and the CWA grants program. The applications section provides three Illustrative Examples related to the general subject of the Clean Water Act.