Istanbul is the largest city in Turkey, a megacity, constituting the country's economic, cultural, and historical center. With a population of over 15 million, it is home to 18% of the population of Turkey. Istanbul is among the largest cities in Europe and in the world by population. It is a city on two continents; about two-thirds of its population live in Europe and the rest in Asia. Istanbul straddles the Bosphorus – one of the world's busiest waterways – in northwestern Turkey, between the Sea of Marmara and the Black Sea. Its area of is coterminous with Istanbul Province.
The city developed to become one of the most significant cities in history. Byzantium was founded on the (present-day) Sarayburnu promontory by Greek colonists in the seventh century BC. Over nearly 16 centuries following its reestablishment as Constantinople in 330 AD, it served as the capital of four empires: 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). It was instrumental in the advancement of Christianity during Roman and Byzantine times, before the Ottomans conquered the city in 1453 and transformed it into an Islamic stronghold and the seat of its caliphate. Although the Republic of Turkey established its capital in Ankara, palaces and imperial mosques still line Istanbul's hills as visible reminders of the city's previous central role. The historic centre of Istanbul is a UNESCO World Heritage Site.
Istanbul's strategic position along the historic Silk Road, rail networks to Europe and West Asia, and the only sea route between the Black Sea and the Mediterranean have helped foster an eclectic populace, although less so since the establishment of the Republic in 1923. Overlooked for the new capital during the interwar period, the city has since regained much of its prominence. The population of the city has increased tenfold since the 1950s, as migrants from across Anatolia have flocked to the metropolis and city limits have expanded to accommodate them. The majority of Istanbul’s Turkish citizens are of ethnic Turkish origin, whereas ethnic Kurds represent the largest ethnic minority in the city.
Considered an alpha global city, Istanbul accounts for about thirty percent of Turkey's economy. The Istanbul-İzmit area is one of the main industrial regions in Turkey. In 2025, Euromonitor International ranked Istanbul as the fifth most visited city in the world. Istanbul is home to two international airports, multiple ports, and numerous universities.for airports and ports, see: for universities, see: It is among the top 100 science and technology clusters in the world. The city hosts a large part of Turkish football and sports in general, with clubs such as Galatasaray, Fenerbahçe and Beşiktaş. Istanbul is vulnerable to earthquakes as it is in close proximity to the North Anatolian Fault.
Names
The first known name of the city is Byzantium (, ), the name given to it at its foundation by Megarian colonists around 657 BC. Megarian colonists claimed a direct line back to the founders of the city, Byzas, the son of the god Poseidon and the nymph Ceroëssa. Modern excavations have raised the possibility that the name Byzantium might reflect the sites of native Thracian settlements that preceded the fully-fledged town. Constantinople (; ) comes from the Latin name , after Constantine the Great, the Roman emperor who refounded the city in 324 AD. Constantine had initially called the city New Rome (; ; ). Constantinople remained the most common name for the city in the West until the 1930s, when Turkish authorities began to press for the use of Istanbul in foreign languages. () and were the names used alternatively by the Ottomans during their rule.
The name (; or colloquially ) is commonly held to derive from the Medieval Greek phrase (, ), literally "to the city",Necdet Sakaoğlu (1993/94a): "İstanbul'un adları" ["The names of Istanbul"]. In: Dünden bugüne İstanbul ansiklopedisi, ed. Türkiye Kültür Bakanlığı, Istanbul. and is how Constantinople was referred to by the local Greeks. This reflected its status as the only major city in the vicinity. The importance of Constantinople in the Ottoman world was also reflected by its nickname () meaning the 'Gate to Prosperity' in Ottoman Turkish. An alternative view is that the name evolved directly from "Constantinople", with the first and third syllables dropped. Some Ottoman sources of the 17th century, such as Evliya Çelebi, describe it as the common Turkish name of the time; between the late 17th and late 18th centuries, it was also in official use. The first use of the word () on coinage was in 1730 during the reign of Sultan Mahmud I. In modern Turkish, the name is written as , with a dotted İ, as the Turkish alphabet distinguishes between a dotted and dotless I. In English, the stress is on the first or last syllable, but in Turkish it is on the second syllable. A person from the city is an (plural ); Istanbulite is used in English.
History
Neolithic artifacts, uncovered by archeologists at the beginning of the 21st century, indicate that Istanbul's historic peninsula was settled as far back as the 6th millennium BC. That early settlement, important in the spread of the Neolithic Revolution from the Near East to Europe, lasted for almost a millennium before being inundated by rising water levels. The first human settlement on the Asian side, the Fikirtepe mound, is from the Copper Age period, with artifacts dating from 5500 to 3500 BC, On the European side, near the point of the peninsula (Sarayburnu), there was a Thracian settlement during the early 1st millennium BC. Modern authors have linked it to the Thracian toponym Lygos, mentioned by Pliny the Elder as an earlier name for the site of Byzantium.
The history of the city proper begins around 660 BC, when Greek settlers from Megara established Byzantium on the European side of the Bosporus. The settlers built an acropolis adjacent to the Golden Horn on the site of the early Thracian settlements, fueling the nascent city's economy. The city experienced a brief period of Persian rule at the turn of the 5th century BC, but the Greeks recaptured it during the Greco-Persian Wars. Byzantium then continued as part of the Athenian League and its successor, the Second Athenian League, before gaining independence in 355 BC. Long allied with the Romans, Byzantium officially became a part of the Roman Empire in 73 AD. Byzantium's decision to side with the Roman usurper Pescennius Niger against Emperor Septimius Severus cost it dearly; by the time it surrendered at the end of 195 AD, two years of siege had left the city devastated. Five years later, Severus began to rebuild Byzantium, and the city regained – and, by some accounts, surpassed – its previous prosperity.
Byzantine era
Constantine the Great effectively became the emperor of the whole of the Roman Empire in September 324. Two months later, he laid out the plans for a new, Christian city to replace Byzantium. As the eastern capital of the empire, the city was named Nova Roma; most called it Constantinople, a name that persisted into the 20th century. On 11 May 330, Constantinople was proclaimed the capital of the Roman Empire, which was later permanently divided between the two sons of Theodosius I upon his death on 17 January 395, when the city became the capital of the eastern empire. During the following millennium of Roman history the state was commonly referred to as the "Byzantine Empire".
The establishment of Constantinople was one of Constantine's most lasting accomplishments, shifting Roman power eastward as the city became a center of Greek culture and Christianity. Numerous churches were built across the city, including Hagia Sophia which was built during the reign of Justinian I and remained the world's largest cathedral for a thousand years. Constantine also undertook a major renovation and expansion of the Hippodrome of Constantinople; accommodating tens of thousands of spectators, the hippodrome became central to civic life and, in the 5th and 6th centuries, the center of episodes of unrest, including the Nika riots. Constantinople's location also ensured its existence would stand the test of time; for many centuries, its walls and seafront protected Europe against invaders from the east and the advance of Islam. During most of the Middle Ages, the latter part of the Byzantine era, Constantinople was the largest and wealthiest city on the European continent and at times the largest in the world. Constantinople is generally considered to be the center and the "cradle of Orthodox Christian civilization".
Constantinople began to decline continuously after the end of the reign of Basil II in 1025. The Fourth Crusade was diverted from its purpose in 1204, and the city was sacked and pillaged by the crusaders. They established the Latin Empire in place of the Orthodox Byzantine Empire. Hagia Sophia was converted to a Catholic church in 1204. The Byzantine Empire was restored, albeit weakened, in 1261. Constantinople's churches, defenses, and basic services were in disrepair, and its population had dwindled to a hundred thousand from half a million during the 8th century. After the reconquest of 1261, however, some of the city's monuments were restored, and some, like the two Deesis mosaics in Hagia Sophia and Kariye, were created.
Various economic and military policies instituted by Andronikos II Palaiologos, such as the reduction of military forces, weakened the empire and left it vulnerable to attack. In the mid-14th-century, the Ottoman Turks began a strategy of gradually taking smaller towns and cities, cutting off Constantinople's supply routes and strangling it slowly. On 29 May 1453, after a 55-day siege during which the last Roman emperor, Constantine XI, was killed, Sultan Mehmed II "the Conqueror" captured Constantinople.
Ottoman Empire
Sultan Mehmed declared Constantinople the new capital of the Ottoman Empire. Hours after the fall of the city, the sultan rode to the Hagia Sophia and summoned an imam to proclaim the shahada, converting the grand cathedral into an imperial mosque due to the city's refusal to surrender peacefully. Mehmed declared himself as the new Kayser-i Rûm, the Ottoman Turkish equivalent of the Caesar of Rome, and the Ottoman state was reorganized into an empire.
Following the capture of Constantinople, Mehmed II immediately set out to revitalize the city. Cognizant that revitalization would fail without the repopulation of the city, Mehmed II welcomed everyone–foreigners, criminals, and runaways– showing extraordinary openness and willingness to incorporate outsiders that came to define Ottoman political culture.Inalcik, Halil. "The Policy of Mehmed II toward the Greek Population of Istanbul and the Byzantine Buildings of the City." Dumbarton Oaks Papers 23, (1969): 229–49. p. 236 He also invited people from all over Europe to his capital, creating a cosmopolitan society that persisted through much of the Ottoman period. Revitalizing Istanbul also required a massive program of restorations, of everything from roads to aqueducts. Like many monarchs before and since, Mehmed II transformed Istanbul's urban landscape with the wholesale redevelopment of the city center. There was a huge new palace to rival, if not overshadow, the old one, a new covered market (still standing as the Grand Bazaar), porticoes, pavilions, walkways, as well as more than a dozen new mosques. Mehmed II turned the ramshackle old town into something that looked like an imperial capital.