For over a decade, the regulation of nutrients (nitrogen and phosphorus) under the Clean Water Act has been a focus of the U.S. Environmental Protection Agency's (EPA's) attention. Now, it is a focus of litigation. Hypoxia in the Gulf of Mexico, algal blooms in Florida, and reduced aquatic life in the Chesapeake Bay have spawned three very different lawsuits, each shedding light on different issues of administrative law and a different stage of the CWA's process for protecting water quality. The one broad issue these cases share is that—at their core—all three stem from fundamental disputes about when EPA may (or must) step in to take action that the CWA reserves in the first instance (or solely) to the states. At the core of each case is a dispute about the contours of the CWA's system of cooperative federalism.