Technical Abstract: Phosphorus (P), an essential element for plant and animal production, can accelerate eutrophication, which has become one of the most ubiquitous water quality impairments in the U.S. This has forced many states to consider the development of recommendations for P applications and watershed management based on the potential for P loss in agricultural runoff. In response, a P Index has been developed to rank field vulnerability to P loss so that high risk areas may be identified for site-specific management. This publication documents the rationale behind the P Index, approach and calculations used, and interpretations of risk assessments, for the U.S. Environmental Protection Agency's final nutrient management planning strategy for Concentrated Animal Feeding Operations (http://epa.gov/waterscience/guide/cafo/). The index accounts for and ranks transport (erosion, runoff, leaching, and landscape position) and source factors (added fertilizer and manure, soil P) controlling P loss and identifies sites where the risk of P movement is expected to be higher than others. Fields at high risk are those where areas of high P application or soil P coincide with zones of active surface runoff or erosion. A P Index has been developed to rank field vulnerability to P loss so that high risk fields may be identified for site-specific management. The Index provides a framework that can be regionally adapted to prevailing topography, geology, and climatic conditions and requires only readily available data.