The largest city of Turkey, in the northwest part of the country on both sides of the Bosporus at its entrance into the Sea of Marmara. Founded c. 660 B.C. as Byzantium, it was renamed Constantinople in A.D. 330 by Constantine the Great, who made it the capital of the Eastern Roman, or Byzantine, Empire. The city was sacked by Crusaders in 1204 and taken by the Turks in 1453. Istanbul was chosen as the official name in 1930.

Istanbul is located in the north-west Marmara Region of Turkey. It encloses the southern Bosporus which places the city on two continents; the western portion of Istanbul is in Europe, while the eastern portion is in Asia. The city boundaries cover a surface area of 1,830.92 square kilometres (707 sq mi), while the metropolitan region, or the Province of Istanbul, covers 6,220 square kilometres.

Istanbul, on both sides of the Bosporus at its entrance into the Sea of Marmara. One of the great historic cities of the world, Istanbul is the chief city and seaport of Turkey as well as its commercial, industrial, and financial center. Manufactures include textiles, glass, shoes, motor vehicles, ships, and cement. The European part of Istanbul is the terminus of an international rail service (formerly called the Orient Express), and at Haydarpasa Station, on the Asian side, begins the Baghdad Railway. Yesilkoy International Airport is nearby.

The city is visited by many tourists and is a popular resort. The environs of Istanbul, particularly the villas, gardens, castles, and small communities along the Bosporus, are famed for their beauty. The part of Istanbul corresponding to historic Constantinople is situated entirely on the European side. It rises on both sides of the Golden Horn, an inlet of the Bosporus, on one of the finest sites of the world, and like Rome is built on seven hills. Several miles of its ancient moated and turreted walls are still standing. Outside the walls and N of the Golden Horn are the commercial quarter of Galata, originally a Genoese settlement; the quarter of Beyoglu (formerly Pera), which under the Ottoman sultans was reserved for foreigners and their embassies; and Haskoy, the Jewish quarter.

The historic peninsula of old Istanbul corresponds approximately to the extent of Constantinople in the 15th century; it comprises the districts of Eminonu and Fatih. This area lies on the southern shores of the Golden Horn, which separates the old city center from the northern and younger parts of the European side. The historic peninsula ends with the Theodosian Land Walls in the west. The peninsula is surrounded by the Sea of Marmara on the south and the entrance of the Bosphorus on the east.

North of the Golden Horn are the historical Beyoglu and Besiktas districts, where the last Sultan's palace is located, followed by a chain of former villages such as Ortakoy and Bebek along the shores of the Bosphorus. On both the European and Asian sides of the Bosphorus, wealthy Istanbulites built luxurious chalet mansions, called yali, which were used as summer residences.

The districts of Uskudar (ancient Chrysopolis) and Kadikoy (ancient Chalcedon) which are located on the Asian side were originally separate cities (like the district of Beyoglu (medieval Pera) on the European side also used to be.) These cities have eventually been absorbed by Istanbul and have become its districts. Today, the Asian side of the city has numerous modern residential areas and business districts, and is home to around one-third of Istanbul's population.

Istanbul has two international airports: The larger one is the Ataturk International Airport located in the Yesilkoy district on the European side, about 24 kilometres (15 mi) west from the city center. When it was first built, the airport was situated at the western edge of the metropolitan area but now lies within the city bounds. The smaller one is the Sabiha Gokcen International Airport located in the Kurtkoy district on the Asian side, close to the Istanbul Park GP Racing Circuit. It is situated approximately 20 kilometres (12 mi) east of the Asian side and 45 kilometres (28 mi) east of the European city center.

Istanbul has numerous historic shopping centers, such as the Grand Bazaar (1461), Mahmutpasa Bazaar (1462) and the Egyptian Bazaar (1660). The first modern shopping mall was Galleria Atakoy (1987), which was followed by dozens of others in the later decades, such as Akmerkez (1993) which is the only mall to win both "Europe's Best" and "World's Best" awards by the ICSC; Metrocity (2003); Cevahir Mall (2005) which is the largest mall in Europe; and Kanyon Mall (2006) which won the 2006 Cityscape Architectural Review Award for its interesting design. Istinye Park (2007) and City's Nisantasi (2008) are two new malls which target high-end consumers and are almost exclusively dedicated to world-famous fashion brands.

There are many night clubs, pubs, restaurants and taverns with live music in the city. The night clubs, restaurants and bars increase in number and move to open air spaces in the summer. The areas around Istiklal Avenue and Nisantasi offer all sorts of cafés, restaurants, pubs and clubs as well as art galleries, theaters and cinemas. Babylon and Nu Pera in Beyoglu are popular night clubs both in the summer and in the winter.

The most popular open air summer time seaside night clubs are found on the Bosporus, such as Sortie, Reina and Anjelique in the Ortakoy district. Q Jazz Bar in Ortakoy offers live jazz music in a stylish environment.

Venues such as Istanbul Arena in Maslak and Kurucesme Arena on the Bosporus frequently host the live concerts of famous singers and bands from all corners of the world. Parkorman in Maslak hosted the Isle of MTV Party in 2002 and is a popular venue for live concerts and rave parties in the summer.