Onondaga Lake, NY, has been described as the most polluted lake in the United States. This medium size (surface area of 12 km2and mean depth of 10.9 m), rapidly flushed (average of 3.9 flushes/y), urban lake has received large quantities of domestic and industrial waste associated with development of the Syracuse area. Selected features of the history of development of the area, including municipal and industrial inputs to the lake, are reviewed. Presently about 20% of the inflow to die lake is municipal wastewater effluent Standards for dissolved oxygen, fecal coliform, free ammonia, nitrite, clarity, and mercury concentration in fish flesh are violated routinely in the lake, a state guidance value for total phosphorus concentration is exceeded annually, and the lake's stratification/mixing regime and littoral zone have been impacted. Enforcement actions, now underway against die primary sources of municipal and industrial waste, are described. The design of die research program for die lake is reviewed, and the role subsequent articles in this issue play in developing a management strategy for remediation is described.