The paleoecology of the Niagaran reefs in the Great Lakes area, constituting the southwestern extremity of widespread archipelagic development in the Niagaran sea, is investigated. The aspects examined encompass, in the area of the physical environmental framework of the reefs, the paleogeographic setting, the nature of the reef foundations, sources and ecologic implications of turbidity, and prevailing wave and current direction. Structural components and growth dynamics of the reefs are analyzed. In the biologic area, the analyses encompass the definition of the reef biota in terms of the upgrowth stages of reefs from quiet-water depth to wave resistance, their relations to the inter-reef biota, habits, and functional morphologic adaptation of the reef organisms and observed evolutionary changes. Brief consideration is given to a gross comparison of the Niagaran with the present-day reef biota.