Decreasing the potential delivery of phosphorus in fertilizers or animal waste to surface and groundwater requires a knowledge of phosphorus's fate and transport mechanisms. The USDA Natural Resources Conservation Service has recently mandated each state to develop an assessment tool to estimate P transport to water bodies. The objective of this paper is to describe the processes involved with P transport to surface and groundwaters that must be accounted for in practical methods used to quantify the potential for P loss. Mechanistic models to assess P loss should account for: 1) P adsorbed to eroding sediments, 2) soluble P in runoff water, 3) soluble P in leaching water, and 4) P losses related to specific P sources. With sediment-bound P, the adsorbed P content in the eroded soil mass at the field edge must be quantified, whereas runoff volume and P concentration are needed to estimate soluble P loss in runoff water. Estimating P leaching potential requires calculation of drainage water volume and P concentration. When P is applied in animal waste, the specific source influences both soluble and particulate P loss because of differences in P solubility between waste types. In addition, the effects of conservation practices and other technologies on decreasing sediment, soluble, and leached P need to be included. Using these methods, a practical, quantitative P loss assessment tool can be developed that will enable technical service providers and other practitioners to estimate potential P loss and design best-management practices for land-applied waste management systems in order to minimize P transport to surface and groundwater.