Maryland and Delaware’s phosphorus (P) site index was released in 2002 with the objective of guiding management practices and P application to reduce the risk of P transport from agricultural fields to surface water. This tool was designed to rate the relative risk of P transport using site and source characteristics. Over the past four years researchers at the Universities of Delaware and Maryland have worked together, with input from other regional experts, to revise the current P site index to include the newest science available. In large part this revision was guided by evaluation of the index’s performance using a database of approximately 9,000 indexes run in Maryland.  In addition, trends in nutrient balances exhibited in Delaware and Maryland since the institution of mandatory nutrient management, including use of the P site index, were considered in evaluating index performance. Evaluation of the 2002 index, development of the revised P site index, and validation of the new index will be discussed.