Within the framework of a European Union-funded project, a comprehensive questionnaire has been prepared and a survey has been realized among the already existing urban wastewater treatment plants of the country with the aim of investigating the reuse possibility of treated effluents. This study forming one of the project milestones, aimed at putting forth the existing situation of the urban wastewater treatment plants of the Greater Metropolitan Istanbul which is one of the most crowded cities of the world. The current situation revealed that as the city has scarce available land and is surrounded by sea, primary treatment is favored. Some of the plants own biological treatment units where the effluents comply with the discharge to receiving water standards. Most of the plants serve to populations over 100 000 and face operational problems Such as lack of automation, clogging of pipes, old-fashioned equipment and working over-capacities. The Water and Sewerage Administration of Istanbul (ISKI) has to supply high quality drinking water to the inhabitants of the city and is responsible for the treatment of the urban wastewater. ISKI has started to initiate not only secondary but also advanced biological treatment units to the already existing plants and urges the companies in charge of their construction to operate the plants for 20 years.