An Area of Concern (AOC) is a location where environmental quality is degraded compared to other areas in the Great Lake Basin resulting in the impairment of beneficial uses. A total of 43 AOCs were identified as a result of Annex 2 of the Great Lakes Water Quality Agreement (GLWQA).The Canada-United States GLWQA identifies 14 beneficial uses that must be restored in order to remove the designation as an Area of Concern. A beneficial use is defined as the ability of living organisms (including humans) to use the Great Lakes Basin Ecosystem without adverse consequences. A Beneficial Use Impairment (BUI) is a condition that interferes with the enjoyment of a water use. Each BUI has a set of locally-defined delisting criteria that are specific, measurable, achievable, and scientifically-defensible. The Remedial Action Plan (RAP) is administered locally in accordance with the Canada-U.S. Great Lakes Water Quality Agreement (GLWQA) and the Canada-Ontario Agreement (COA). The RAP is an ongoing collaborative effort implemented by federal, provincial, and local governments as well as industry and public partners. There are 3 key stages of the RAP: Stage 1 is a detailed description of the environmental problem; Stage 2 identifies remedial actions and options; Stage 3 is the final document providing evidence that the beneficial uses have been restored and the AOC can be “delisted”. The Toronto and Region Area of Concern extends along the northern shoreline of Lake Ontario from the Rouge River in the east to Etobicoke Creek in the west. The 2000 km2 (200 000 ha) area includes the Toronto waterfront and 6 watersheds: Etobicoke Creek, Mimico Creek, Humber River, Don River, Highland Creek and Rouge River. The drainage basin of these watersheds makes the Area of Concern a study in contrasts: more than 40% of the area is still rural and contains one of the world’s largest natural parks in an urban/agricultural setting; at the same time, more than three million people live in the Area of Concern and the City of Toronto is in the centre of the most densely urbanized area in Canada. The Toronto region was designated as an AOC in 1986 because a review of available data indicated that water quality and environmental health were severely degraded. Several centuries of agriculture and urban development have dramatically reshaped the natural environment of the Toronto and Region AOC. Contaminants from stormwater runoff and melting snow from the area’s six watersheds create serious impacts in Lake Ontario. Overflows of stormwater mixed with raw sewage are a serious problem following heavy rains in the lower portions of the Don and Humber Rivers and along the waterfront. Spills, road runoff and chemical input to sewers from industries and residences also contribute to poor water quality. In the Toronto and Region AOC’s Remedial Action Plan (RAP) report, Clean Waters, Clear Choices: Recommendations for Action (1994) eight beneficial uses were identified as impaired and three were identified as requiring further assessment. For more information visit https://www.canada.ca/en/environment-climate-change/services/great-lakes-protection/areas-concern/toronto-region.html