Istanbul, historically known as Byzantium and Constantinople, is the largest city in Turkey and the country's economic, cultural and historic center. The city straddles the Bosphorus strait, and lies in both Europe and Asia, with a population of over 15 million residents, comprising 19% of the population of Turkey. Istanbul is the most populous city in Europe, and the world's fifteenth-largest city. Founded as Byzantion by Megarian colonists in 660 BCE, and renamed as Constantinople in 330 CE, the city grew in size and influence, becoming a beacon of the Silk Road and one of the most important cities in history. It served as an imperial capital for almost sixteen centuries, during the Roman/Byzantine (330–1204), Latin (1204–1261), Byzantine (1261–1453), and Ottoman (1453–1922) empires.

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 BCE. 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 BCE, On the European side, near the point of the peninsula (Sarayburnu), there was a Thracian settlement during the early 1st millennium BCE. 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 BCE, when Greek settlers from Megara established Byzantium on the European side of the Bosphorus. 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 BCE, 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 BCE. Long allied with the Romans, Byzantium officially became a part of the Roman Empire in 73 CE. 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 CE, 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.

Rise and fall of Constantinople and the Byzantine Empire

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 Roman (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 the Great 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 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 Sofia and Kariye, were created.

Various economic and military policies instituted by Andronikos II, 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 an eight-week siege (during which the last Roman emperor, Constantine XI, was killed), Sultan Mehmed II "the Conqueror" captured Constantinople and declared it the new capital of the Ottoman Empire. Hours later, the sultan rode to the Hagia Sophia and summoned an imam to proclaim the Islamic creed, converting the grand cathedral into an imperial mosque due to the city's refusal to surrender peacefully. Mehmed declared himself as the new "Kaysar-i Rûm" (the Ottoman Turkish equivalent of Caesar of Rome) and the Ottoman state was reorganized into an empire.

Ottoman Empire and Turkish Republic eras

Following the conquest 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. 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 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.

Social hierarchy was ignored by the rampant plague, which killed the rich and the poor alike in the sixteenth century. Money could not protect the rich from all the discomforts and harsher sides of Istanbul. Although the Sultan lived at a safe remove from the masses, and the wealthy and poor tended to live side by side, for the most part Istanbul was not zoned as modern cities are. Opulent houses shared the same streets and districts with tiny hovels. Those rich enough to have secluded country properties had a chance of escaping the periodic epidemics of sickness that blighted Istanbul.

The Ottoman Dynasty claimed the status of caliphate in 1517, with Constantinople remaining the capital of this last caliphate for four centuries. Suleiman the Magnificent's reign from 1520 to 1566 was a period of especially great artistic and architectural achievement; chief architect Mimar Sinan designed several iconic buildings in the city, while Ottoman arts of ceramics, stained glass, calligraphy, and miniature flourished. The population of Constantinople was 570,000 by the end of the 18th century.

A period of rebellion at the start of the 19th century led to the rise of the progressive Sultan Mahmud II and eventually to the Tanzimat period, which produced political reforms and allowed new technology to be introduced to the city. Bridges across the Golden Horn were constructed during this period, and Constantinople was connected to the rest of the European railway network in the 1880s. Modern facilities, such as a water supply network, electricity, telephones, and trams, were gradually introduced to Constantinople over the following decades, although later than to other European cities. The modernization efforts were not enough to forestall the decline of the Ottoman Empire.

Sultan Abdul Hamid II was deposed with the Young Turk Revolution in 1908 and the Ottoman Parliament, closed since 14 February 1878, was reopened 30 years later on 23 July 1908, which marked the beginning of the Second Constitutional Era. A series of wars in the early 20th century, such as the Italo-Turkish War (1911–1912) and the Balkan Wars (1912–1913), plagued the ailing empire's capital and resulted in the 1913 Ottoman coup d'état, which brought the regime of the Three Pashas.

The Ottoman Empire joined World War I (1914–1918) on the side of the Central Powers and was ultimately defeated. The deportation of Armenian intellectuals on 24 April 1915 was among the major events which marked the start of the Armenian Genocide during WWI. Due to Ottoman and Turkish policies of Turkification and ethnic cleansing, the city's Christian population declined from 450,000 to 240,000 between 1914 and 1927. The Armistice of Mudros was signed on 30 October 1918 and the Allies occupied Constantinople on 13 November 1918. The Ottoman Parliament was dissolved by the Allies on 11 April 1920 and the Ottoman delegation led by Damat Ferid Pasha was forced to sign the Treaty of Sèvres on 10 August 1920.

Following the Turkish War of Independence (1919–1922), the Grand National Assembly of Turkey in Ankara abolished the Sultanate on 1 November 1922, and the last Ottoman Sultan, Mehmed VI, was declared persona non grata. Leaving aboard the British warship HMS Malaya on 17 November 1922, he went into exile and died in Sanremo, Italy, on 16 May 1926. The Treaty of Lausanne was signed on 24 July 1923, and the occupation of Constantinople ended with the departure of the last forces of the Allies from the city on 4 October 1923. Turkish forces of the Ankara government, commanded by Şükrü Naili Pasha (3rd Corps), entered the city with a ceremony on 6 October 1923, which has been marked as the Liberation Day of Istanbul (Turkish: İstanbul'un Kurtuluşu) and is commemorated every year on its anniversary. On 29 October 1923 the Grand National Assembly of Turkey declared the establishment of the Turkish Republic, with Ankara as its capital. Mustafa Kemal Atatürk became the Republic's first President. According to historian Philip Mansel:

after the departure of the dynasty in 1925, from being the most international city in Europe, Constantinople became one of the most nationalistic....Unlike Vienna, Constantinople turned its back on the past. Even its name was changed. Constantinople was dropped because of its Ottoman and international associations. From 1926 the post office only accepted Istanbul; it appeared more Turkish and was used by most Turks.

A 1942 wealth tax assessed mainly on non-Muslims led to the transfer or liquidation of many businesses owned by religious minorities. From the late 1940s and early 1950s, Istanbul underwent great structural change, as new public squares, boulevards, and avenues were constructed throughout the city, sometimes at the expense of historical buildings. The population of Istanbul began to rapidly increase in the 1970s, as people from Anatolia migrated to the city to find employment in the many new factories that were built on the outskirts of the sprawling metropolis. This sudden, sharp rise in the city's population caused a large demand for housing, and many previously outlying villages and forests became engulfed into the metropolitan area of Istanbul.