Despite the success of efforts to reduce phosphorus (P) pollution from point sources, P from non-point agricultural sources remains a vexing problem with many U.S. water bodies having impairments. Key to solving the P pollution puzzle is to take stock of progress to date, the puzzle pieces available, and the gaps to be filled. In this paper, we synthesize the state of knowledge on P pollution, discuss the state of existing public programs, and review economists’ contributions to informing P pollution policies. We review the water quality valuation literature, identifying limitations in the linkages to policy-relevant environmental quality metrics. We examine how and why P is used agriculturally, along with recent advances in market-based policy design and field testing. We survey new knowledge in biology and engineering, including improved understanding of the fate and transport of P. In light of recent learning and persistent knowledge gaps, we recommend directions for economic research to add needed pieces to the puzzle of how to protect our water bodies. Puzzle gaps meriting attention include mechanisms to target public funds more effectively in voluntary abatement programs, policy design for emerging mitigation technologies, new ways to implement performance-based policies, means to leverage social norms and behavioral cues, changes in the “pay-the-polluter” paradigm, and application of state-of-the-art evaluation methods to conservation programs. Beyond the realm of public policy lies that of private supply chains, where establishment of environmental standards holds additional promise. Rich research opportunities exist for economists in tandem with biologists, engineers, and others.