Despite advances in modeling, such as graphical user interfaces, the use of GIS layers, and databases for developing input files, the approaches to modeling phosphorus (P) have not changed since their initial development in the 1980s. Current understanding of P processes has evolved and this new information needs to be incorporated into the current models. Filling this need, Modeling Phosphorus in the Environment describes basic approaches to modeling P, how the current models implement these approaches, and ways to improve them. The book sets the scene with a review of general approaches to modeling runoff and erosion, P in runoff, leaching of P, stream processes that affect P, and an examination of the important issue of model uncertainty. It describes state-of-the-science watershed-scale P transport models including dynamic semi-disturbed models, models of intermediate complexity, and two lumped models. Phosphorus Indexes (PIs) represent one end of the modeling spectrum and the booktakes a comprehensive look at PIs developed in each state, and illustrates some of the problems encountered when incorporating PIs into farm-scale manure management software. The book discusses monitoring data, which is critical for calibrating models, and concludes with suggestions for improving the modeling of P.