This guidance presents a process for screening and ranking combined sewer systems (CSSs) with combined sewer overflows (CSOs) that have adverse impacts on water quality, aquatic life, or human health. Its primary purpose is to give the National Pollutant Discharge Elimination System (NPDES) permitting authorities (EPA Regions and approved NPDES States) a method of prioritizing the issuance of NPDES permits to communities with CSOs. A secondary purpose is to give communities with multiple CSOs discharging to multiple receiving water bodies, a tool for ranking CSOs. This will give the communities a basis for allocating resources to eliminate or control, in accordance with the policy, those CSOs with the most significant impacts, and maximize the environmental benefits achieved for the resources expended. It can also help target monitoring needs. This ranking process relies primarily on information readily available for most CSSs, such as a general knowledge of known or expected impacts from CSOs; estimates of CSO flows and their characteristics; and receiving water characteristics (e.g., stream, lake, marine, or estuarine waters).