Selecting the appropriate level of CSO control is a function of capital and operation costs and the degree to which the receiving environment must be protected. Selecting too high a level of protection increases costs un-necessarily while selecting too low a level of protection also has a high cost. To assist in prioritizing implementation of CSO control projects and to help realize the greatest benefit from the expenditures, King County, Washington performed a Water Quality Assessment (WQA) to compare alternative control methods and to prioritize projects. This paper discusses the development of the WQA, use of the findings in initial project selection, and describes the on-going program to improve on the assessment.