INTRODUCTION Stormwater has long been recognized as a substantial contributor to water quality impairments. Development has increased the area of impervious surfaces and disrupted the natural flow path for precipitation. In developed areas, large volumes of untreated stormwater runoff increase erosion and pollutant transport to surface waters. Regulators have designed programs to address the water quality impacts of stormwater and regulated entities are in the process of figuring out how to comply with these measures. Financial burden often is cited as a major reason for slow implementation and lack of compliance with stormwater regulations (NRC, 2009). Regulated entities have argued that the permit requirements are overly burdensome and unrealistic; however, it is still too early to determine the full financial burden of stormwater regulation. Although the regulations were enacted several years ago (and continue to evolve), many entities are still in the early phases of the implementation process and are trying to determine how to integrate stormwater controls into existing infrastructure. In addition, municipalities often have limited information about the cost of retrofits. The cost of compliance with stormwater regulation is one of the major unknowns facing municipalities and other regulated stormwater dischargers. Regulated entities should expect to incur high costs associated with stormwater controls, especially in areas that are already highly developed. Exactly how high these costs might be is uncertain. This makes it very difficult for decision makers to plan and budget for stormwater controls. As a result, many municipalities have delayed implementing these measures despite increasing pressure from regulators. Entities soon will have to begin financing and implementing stormwater controls. This paper illustrates the lack of, and uncertainty with, cost data available to planners and decision makers and provides an example where a regulated entity applied a localized analysis to cost effectively achieve stormwater reductions and compliance goals.