This chapter is for the Second Edition of Coastal and Ocean Law, published by the American Bar Association's Section on Environment, Energy, and Resources. It comprehensively reviews the application of the Clean Water Act in the coastal zone and at sea. For example, the chapter details the regulatory significance of the Act's recognition of three marine zones -- the territorial sea, the contiguous zone, and the ocean -- and explains how the Clean Water Act applies to discharges from non-military ships, vessels of the Armed Forces, and "other floating craft" as well as land-based sewage discharges and discharges from offshore oil and gas platforms. The Second Edition version of this chapter also highlights developments in this area of law since 2006. For example, the chapter highlights the EPA's newest (2012) National Coastal Condition Report. Moreover, it details the relatively rapid change in the law governing ballast water and other discharges from ships, examines the Chesapeake Bay multi-state TMDL and the litigation over the "dead zone" in the Gulf of Mexico, and examines states' increasing use of the Act's "No Discharge Zone" provisions, especially California's 2012 establishment of its entire coastline as a "No Discharge Zone."