Modifying animal diets to reduce the excretion of phosphorus is rapidly becoming an accepted, effective, and economical best management practice for water quality protection. A considerable body of literature indicates that modifying poultry and dairy diets can reduce the amount of P excreted by from 20 to 50%. This greatly decreases the potential for farm or watershed surpluses of P and the potential for P buildup in soils to values of environmental concern. Paralleling efforts by animal scientists to develop improved diets has been a multi-disciplinary effort to more accurately characterize the exact inorganic and organic species of P present in manures. Advances in analytical methodology to characterize P, such as ³¹P NMR and XANES spectroscopy, have provided insights into how changing diets alters, or does not alter, P speciation in manures. Knowledge of the P species present in manures is important to our efforts to better predict the potential for P loss to water from manured soils. This presentation reviews the literature on the use of new analytical methods, alone, or in combination with more traditional techniques (e.g., chemical fractionation) to determine P speciation in manures. Implications of an improved understanding of P speciation in animal manures produced from modified diets will be discussed and related to watershed-scale efforts to protect and improve water quality.