This paper reports on the progress of an US EPA funded cooperative research program with the American Society of Civil Engineers (ASCE) to develop a more useful set of data on the effectiveness of stormwater best management practices (BMPs) used to reduce pollutant discharges from urban development. BMP performance data gathered at a particular site should not only be useful for that site, but also need to be useful for comparing the performance of similar and different types of BMPs at other locations so that better decisions regarding BMP selection and design can be made. Most BMP effectiveness studies in the past have provided data that is difficult, if not impossible, to use in comparing BMP design effectiveness and in the selection amongst individual BMP types to meet desired goals. This paper describes some of the comparability problems encountered between different BMP effectiveness studies. Also discussed are considerations that affect data transferability, such as methods used for determining efficiency and statistical significance. It outlines the efforts used to establish and analyze the currently available data and proposes protocols for future analyses, when more studies that have data are available. Finally, it recommends that effluent quality is probability a much more robust measure of BMP effectiveness and performance than the currently used “percent removal” metrics.