Istanbul (historically Byzantium and Constantinople) is the largest city of Turkey, home to a population of 13,483,052 in 2011. A megacity, it is the nation's cultural, economic, and financial center.

Located in the northwest of the country, it lies on the Bosphorus strait and encompasses the natural harbour known as the Golden Horn. Extending both on the European (Thrace) and Asian (Anatolia) sides of the strait, Istanbul is the only metropolis in the world situated on two continents. It covers 39 districts of Istanbul province. The greater Istanbul metropolitan area held 18% of Turkey's population in 2010. It ranks as the world’s 7th fastest growing metropolitan area in 2011.

During its long history, Istanbul has served as the capital of the Roman Empire (330–395), the Eastern Roman (Byzantine) Empire (395–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). When the new Republic of Turkey was proclaimed in 1923, Ankara was chosen as its capital.

Istanbul's historic areas were added to the UNESCO World Heritage List in 1985. It was named a joint European Capital of Culture for 2010 and the European Capital of Sports for 2012. Istanbul is currently bidding to host the 2020 Summer Olympics.