Urban Wastewater Treatment and Marine Outfall Facilities are being planned in accordance with Istanbul Water Supply and Wastewater Treatment and Disposal Master Plan prepared for Istanbul Water and Sewerage Administration (ISKI) in 1999. According to the Master Plan, ISKI initiated in 1996 the water quality monitoring program of receiving water bodies conducted by the Istanbul University (IU) Institute of Marine Sciences and Management and Istanbul Technical University (ITU) Environmental Engineering Department. The monitoring program has been carried out for 10 years in 28 stations in the marine environment. In this paper, the results of experimental investigations conducted for parameters including dissolved oxygen, total organic carbon (TOC), particulate organic carbon (POC), Total-N, Total-P, chlorophyll a, structure of benthic community, turbidity and coliforms have been critically analysed taking into consideration the marine outfall and surface water discharges from Istanbul's major wastewater treatment plants together with the transboundary pollution sourced by the Black Sea. The studies reveal that the water quality in the Marmara Sea is deteriorated more than anticipated before, and it is a necessity to develop international cooperation aimed for taking more effective measures in the area.