Technical Abstract: Non-point source pollution by agricultural phosphorus (P) runoff is an environmental concern. An important source of P runoff is surface-applied manure. Research has improved understanding of processes controlling manure P loss in runoff, but this improved understanding has not been adequately translated into computer models used to assess the environmental impact of agriculture. Current versions of most models do not simulate surface application of manure and associated P loss in runoff, which research has clearly shown to be a critical process. Key aspects of manure processes include: i) infiltration of manure P into soil at application, ii) release of manure P during storms and transfer to runoff, and iii) manure P transformations between storms, including decomposition, assimilation into soil, and P mineralization. The major research gaps: i) manure P infiltration into soil at application and availability to runoff, ii) how manure properties control P release, iii) the rate of manure decomposition as a function of manure type or management, iv) assimilation of manure and P into soil as a function of soil fauna, climate, and manure characteristics, v) manure P mineralization as a function of climate, animal species, diet, or manure management, vi) manure P processes for grazing animals. Advances in modeling manure P transformations and loss in runoff have been made only recently, and more progress is needed. Thus, research on animal manures and the environmental impact of P needs to shift towards developing and improving models.