The metropolis of Istanbul has faced water supply challenges throughout history; however the situation escalated in the past decade with rapid population growth. Water demand of an ever-growing city could only be solved by transporting water from surrounding water basins as far as 190 km away from the city border. Moreover, imbalanced distribution of water resources and imbalanced water demand on the European and Asian side of the city was resolved by transporting water from Asian side to the European side, by laying water transmission pipes under the Bosphorus Strait and by constructing a tunnel passing 130 m under the Strait. Another difficulty that required challenging solutions was the protection of water reservoirs of the city, hence illegal settlements arose in the reservoir watershed zones due to fast urban growth and migration from rural areas. Discharge of wastewater from illegal settlements caused not only algae growth in water treatment plant (WTP) of one of the largest reservoirs of Istanbul, but also eventual death of algae was a major setback due to anaerobic conditions created in the reservoir. The problem was solved by tertiary treatment of the wastewater and by diverting the effluent through a tunnel away from the reservoir. The aim of this paper is to provide an insight into setbacks faced in a metropolis of 14 million and technical solutions provided against the pollution of reservoirs.