The Delaware Estuary Monitoring Report summarizes data and monitoring program developments for the 1999-2003 calendar years. This report was prepared by the Delaware River Basin Commission (DRBC) Monitoring Coordinator under the direction of the DRBC Monitoring Advisory Committee. It fulfills a program element of the Comprehensive Conservation and Management Plan (CCMP) for the Delaware Estuary. The Delaware Estuary is an interstate watershed that occupies over 6,700 square miles in three states: Delaware, New Jersey, and Pennsylvania. It extends 134 miles from the mouth of the Delaware Bay between Cape May, New Jersey, and Cape Henlopen, Delaware upstream through Wilmington, Camden, and Philadelphia to the falls of the Delaware River at Trenton, New Jersey. Its tributary watersheds drain urban, suburban, and rural communities. Many industrial areas affect in different ways the water quality and habitat in the Delaware Estuary. The Delaware Estuary is a major transportation corridor and home of the world's largest freshwater port, the Philadelphia port complex, and the second largest oil port in the United States. The Delaware Bay also handles about 85% of the East Coast's oil imports and serves six major refineries. In 2002, ports along the Delaware River and its tributaries handled about 118 million tons of imports and 75.4 million tons of exports.