Picture a river winding through farmland. After a heavy rain, brown water loaded with soil and fertilizer rushes into the stream, feeding a bright green algae bloom downstream. At the same time, a metal pipe from a nearby factory pours out wastewater that looks clean but still carries dissolved nutrients. Which source is easier to regulate? That question sits at the heart of water quality policy. In this chapter, we explore why early laws zeroed in on the factory pipe, why the diffuse runoff from farms is now our biggest water-quality headache, and how clever market-based ideas — like trading pollution clean-up across watersheds — promise cleaner water at lower cost.