Partial Summary: To evaluate rain barrels as a means of detaining runoff from the stormwater system and thus reducing Combined Sewer Overflow (CSO) flows, sixteen 75-gallon rain barrels were installed at ten representative, privately owned detached, semi-detached, and rowhouses in the NW, NE and SE quadrants of the District of Columbia, mostly within the CSO portion of the city. Based on rooftop area, runoff volume from a 0.19” rainfall and the 75-gallon barrel capacity, approximately 0.21 acres out of the total 0.25 acres of rooftop area was controlled during the rain barrel demonstration project. This represents approximately 82% of the total combined rooftop area of the ten sites. Results show that the rain barrels controlled approximately 27,521 gallons out of a total of 211,950 gallons of runoff during the study period. This represents an 11.5 percent average runoff capture efficiency across all of the sites. The results further indicate that, to achieve a meaningful (e.g. 1,000,000 gallon) reduction in CSO volume, it would be necessary to install rain barrels at approximately 20 percent of rowhouses (i.e., approximately 6,981 units) in the CSO portion of the city at an estimated cost of roughly $1.7 million. The cost effectiveness of rain barrels as an integral part of the Long Term Control Plan stormwater volume control is somewhat questionable at this time because of the large number of 75-gallon barrels required to control a significant percentage of roof area.