This Article, the fourth in a series of five, examines the continuing struggles to define “point source” and “nonpoint source” under the Clean Water Act. State regulation of nonpoint sources is neither pervasive nor robust, and most continuing water pollution problems can be traced primarily to nonpoint sources. EPA should define nonpoint sources by regulation and begin to expand the definition of point source by incorporating established case law and Agency practice to bring more nonpoint sources into the point source definition.