With the attainment of secondary treatment by virtually all municipal discharges in the United States, control of water pollution from combined sewer overflows (CSOs) has assumed a high priority. Accordingly, a national strategy was issued in 1989 which, in 1993, was expanded into a national policy on CSO control. The national policy establishes as an objective the attainment of receiving water quality standards, rather than a design storm/treatment technology based approach. A significant percentage of the CSOs in the U.S. are located along the Ohio River. The states along the Ohio have decided to coordinate their CSO control efforts through the Ohio River Valley Water Sanitation Commission (ORSANCO). With the Commission assigned the responsibility of developing a monitoring approach which would allow the definition of CSO impacts on the Ohio, research by the Commission found that very little information existed on the monitoring and assessment of large rivers for the determination of CSO impacts. It was therefore necessary to develop a strategy for coordinated efforts by the states, the CSO dischargers, and ORSANCO to identify and apply appropriate monitoring approaches. A workshop was held in June 1993 to receive input from a variety of experts. Taking into account this input, a strategy has been developed which sets forth certain approaches and concepts to be considered in assessing CSO impacts. In addition, the strategy calls for frequent sharing of findings in order that the data collection efforts by the several agencies can be mutually supportive and lead to technically sound answers regarding CSO impacts and control needs.