Izmir Bay is one of the great natural areas of the Mediterranean and compares well with similar coastal areas in the world. Izmir which is the main urban area around the Bay is an important industrial, commercial and cultural focal point. It has the second biggest harbour and is the third largest city in the country. The Izmir metropolitan area has experienced rapid population increase which placed an intense pressure on the environment of the bay, housing, infrastructure and social services. The main sources of pollution in the bay are domestic and industrial effluents which account for 50% of the observed organic pollution. Rivers discharging to the Izmir Bay carry mostly industrial discharges. The main forms of pollution in the Izmir Bay are organic pollution, nutrients, pathogenic organisms and toxic material carried by rivers. Scientific evidence suggests if no pollution control measures are taken immediately pollution in the Bay will soon reach a critical level leading to the collapse in the ecosystem. With this understanding Izmir Sewerage and Stream Control Project which foresees the reestablishment of the Bay's water quality was started in 1983. The preliminary assessment of the costs and benefits of the project suggests that the benefits of the proposed pollution control system could significantly outweigh the costs of the control measures by a factor of about eight.