While Constantinople is famous for its strategic location, the triangular peninsula on which the city rested lacked sufficient fresh water sources. An extensive system of aqueducts was built in Late Antiquity to help supply water to the growing population of Constantinople. This system also included a large number of cisterns that stored waters supplied by aqueducts. These cisterns include huge open cisterns or reservoirs, like the Cistern of Aetius, as well as covered or underground cisterns, like the Basilica Cistern. The Aqueduct of Valens and monumental cisterns are among the most impressive Byzantine structures of modern Istanbul. 
Constantinople’s insufficient water sources made cisterns an essential feature of its water supply system. The city had to rely on its Thracian hinterland for water, as the city itself had no rivers, a few small springs and a long dry summer with little rainfall. The Lycus (Lykos), while often called a river, was in fact a small stream emptying into a bay that was later transformed into the Harbor of Theodosius. There were also a few natural springs that later became associated with the Virgin Mary, such as the famous holy spring (hagiasma) at the Church of Blachernai. As none of these water sources provided adequate water for city’s growing population in Late Antiquity (which perhaps was half a million in the 5th-6th century), Constantinople came to acquire an extensive aqueduct system and an unprecedented number of cisterns.  
Constantinople never matched the water supply of the city of Rome, which had eleven aqueducts including the monumental Aqua Claudia. Nevertheless it involved some of the most remarkable engineering of antiquity, including the longest aqueduct system of the ancient world. Originating in Thrace, it brought water from over 120 kilometers away, on perhaps more than 550 km of channels. Recent research has documented more than 200 cisterns from the Byzantine era in the city, with most of them being covered cisterns built prior the 7th century. They became central to Constantinople’s water supply system and required significant maintenance. Major cisterns were usually placed on or near the top of hills to supply water to various buildings, such as public baths, palaces, monasteries, and churches. These cisterns stored the water brought in by aqueducts, as there were water shortages during the long dry summers of the region. They were also essential for the security of the city, particularly during sieges. 
There were two main aquiferous areas in the vicinity of Constantinople. One area, now known as Halkalı, was about 15 kilometers west of the city. Another area, now called the Belgrade Forest, has two streams that empty into the Golden Horn. These are probably the first water sources first exploited by the city of Byzantium. The first recorded aqueduct of Byzantium was built by Hadrian (117-138), while Valens (364-378) seems to have completed another aqueduct system that was possibly begun by Constantius II (337-361). The Aqueduct of Valens was necessary as many new parts of Constantine’s new city had a higher elevation than Byzantium and its older aqueduct system. The Aqueduct of Valens was also supplied by the long distance water supply system originating in Thrace. 
As the water supply lines coming from Halkalı and the Belgrade Forest were extensively redeveloped by the Ottomans, it is difficult to determine the surviving Roman and Byzantine features of the system. However there are certain geographical features that allow for a hypothetical course to be proposed. The elevation of this water supply line suggests that the Valen’s line originated at Halkalı and entered the city north of Gate of Charisius (Edirnekapı). This line had a higher elevation that supplied the open cisterns of Aetius and Aspar, crossed the Aqueduct of Valens and terminated in the Cistern of Philoxenos. Hadrian’s line seems to correspond with the main Ottoman water supply channel the Kırkçeşme system that brought water from the Belgrade Forest and entered the city just north of Kaligaria Gate (Eğri Kapı). If this is the case, then it ran north of the Valens line, supplying the large cistern near the Pantokrator Monastery and the Basilica Cistern. It also supplied the area around the Great Palace, the Baths of Zeuxippus and the Hippodrome. While there is no physical evidence of the Hadrian’s line, several channels were discovered within the city walls that have approximately the same elevation of the Aqueduct of Valens, including channels at fora of Constantine and Theodosius.
The water supply of Constantinople decreased in the 7th and 8th century as the Avars cut the Valens line during the Siege of Constantinople of 626, and was only restored around 765 by Constantine V (741-775). It is unclear how serious the damage was done on the Valens line, which could have had disastrous implications for the city’s demographics. While it has generally been argued that the Valens line ceased functioning completely, it also has been suggested that the city continued to be supplied by water sources closer to the city.  The next record work on the water system dates to the reign of Basil II (976-1025). Large amounts of water were required in this period, as the city possibly reached around half a million by the 11th century. Repairs were also made on the aqueducts around 1170 during the reign of Manuel I Komnenos (1043-1080). It seems that the long-distance water system was beyond repair by the 12th century, so the city possibly mostly relied on the water sources from the Belgrade Forest, as it did later during the Ottoman era. The water supply system was not repaired following the Latin conquest of Constantinople in 1204, after which cisterns became the main source of water in the city. Collecting rainwater became much more important after the aqueduct lines were no longer functions. These cisterns were no longer used for the supply or distribution of water during the Ottoman era. For example, many of the surviving covered cisterns were later used in the production of silk during the Ottoman era.
Mehmet II ordered work on the aqueduct system to supply shortly after the conquest of Constantinople in 1453.  It seems that the ruins of Byzantine aqueducts were still prominent when he began to rebuild the water systems, making it unclear how much was restored or rebuilt. Mehmet II redeveloped the water system in Halkalı to supply the Aqueduct of Valens (Bozdoğan Kemeri). He also redeveloped part of the water system in the Belgrade Forest, using the water sources on the southern bank of the Cebecikoy Stream. This system was known as Kırkçeşme (Turkish “forty fountains”)  after a public fountain built by Mehmet near the Aqueduct of Valens. 
The most impressive Ottoman water system was the Kırkçeşme water supply system that was restored and expanded by Mimar Sinan between 1554 and 1563. This system, which brought large quantities of water from the Belgrade Forest on the north side of the city, was the biggest to be built during the Ottoman period. It seems that Suleiman the Magnificent (1520-1566) commissioned this water system after noticing water leaking from an old aqueduct when hunting around the Kağıthane Stream. During his reign increases in Constantinople’s population were causing water shortages in the city. Mimar Sinan’s system collected water from the branches of the Kağıthane Stream in the Belgrade Forest, partly using the old Roman water channels as a guide. This huge water supply system had 55 kilometers of channels and 33 aqueducts, five of which were monumental with several tiers of arches. It also included water distribution basins and a city distribution network with 300 public fountains. Many of its features, such as the aqueducts, settling basins, covered channels, distribution basins, and fountains, had been central to the water systems of Rome and Constantinople centuries earlier. It has been estimated that it supplied the city with 12,000-17,000 cubic meters of water day, depending in part on the season. Staring in the 17th century, dams were built to increase the Kırkçeşme water supply system. It is possible that there was a Byzantine dam built in the 4th century around the site of the Büyük Bent and perhaps the Avyad Dam as well. The Kırkçeşme water supply system can be considered Sinan's greatest work in terms of both scale and cost.

During the reign of Mahmud I (1730-1754), a new aqueduct system known as Taksim supplied Pera (Galata) with water. It consisted of three dams, Valide Bendi (1750), Topuzlu Bendi (1750), and Sultan Mahmut Bendi (1839), along with two aqueducts, the Sultan Mahmut Kemeri and Bahçeköy Kemeri. Taksim Square is named after the Taksim Maksemi, as its water distribution chamber is located there. Water from Terkoz Lake began to supply the city with water in 1874, leading to the modernization of the city's water system.