Storm water is a term used by engineers to describe precipitation. Storm water that does not soak into the ground becomes runoff, which either flows into surface waterways or is channeled into storm sewers.

In the United States, the EPA is charged with regulating stormwater as part of the Clean Water Act. The National Pollution Discharge Elimination System (NPDES) is a set of rules that addresses discharges, including stormwater. Phase I rules were put into effect in 1992, and relate to construction sites larger than 5 acres (20,000 m²), industrial discharges, and large municipalities. Phase II rules came into effect in 2003 and addresses municipalities of 150,000 or more and runoff from construction sites 1 acre (4,000 m²) in size. The changes brought about by Phase II were due mainly to an opinion issued by the 9th Circuit Court of Appeals in the case of NRDC v. EPA (June 4, 1992).