Istanbul had 13.26 million people living in it as of December, 2010, which is 18% of Turkey's population and the 3rd largest metropolitan area in Europe (if its Asian half is counted) after London and Moscow. The city in its administrative limits had 8.8 million residents counted in the latest Turkish census from 2000. Istanbul is a megacity, as well as the cultural, economic, and financial centre of Turkey. It is located on the Bosphorus Strait and encompasses the natural harbour known as the Golden Horn, in the northwest of the country. It extends both on the European (Thrace) and on the Asian (Anatolia) sides of the Bosphorus, and is thereby the only metropolis in the world that is situated on two continents. Istanbul is a designated alpha world city.

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 Republic of Turkey was proclaimed on 29 October 1923, Ankara, which had previously served as the headquarters of the Turkish national movement during the Turkish War of Independence, was chosen as the new Turkish State's capital. Istanbul was chosen as 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.[5] The historic areas of the city were added to the UNESCO World Heritage List in 1985.
The city covers 39 districts of the Istanbul province.

Istanbul is located in northwestern Turkey within the Marmara Region on a total area of 5,343 square kilometers (2,063 sq mi).[note 2] The Bosphorus, which connects the Sea of Marmara to the Black Sea, divides the city into a European side, comprising the historic and economic centers, and an Asian, Anatolian side; as such, Istanbul is one of the two bi-continental cities in Turkey among with Çanakkale. The city is further divided by the Golden Horn, a natural harbor bounding the peninsula where the former Byzantium and Constantinople were founded. In the late-19th century, a wharf was constructed in Galata at the mouth of the Golden Horn, replacing a sandy beach that once formed part of the inlet's coastline. The confluence of the Sea of Marmara, the Bosphorus, and the Golden Horn at the heart of present-day Istanbul has deterred attacking forces for thousands of years and still remains a prominent feature of the city's landscape.

The historic peninsula is said to be built on seven hills, each topped by an imperial mosque, surrounded by 22 kilometers (14 mi) of city walls; the largest of these hills is the site of Topkapı Palace on the Sarayburnu. Rising from the opposite side of the Golden Horn is another, conical hill, where the modern Beyoğlu district is situated. Because of the topography, buildings were once constructed with the help of terraced retaining walls (some of which are still visible in older parts of the city), and roads in Beyoğlu were laid out in the form of steps. Üsküdar on the Asian side exhibits similarly hilly characteristics, with the terrain gradually extending down to the Bosphorus coast, but the landscape in Şemsipaşa and Ayazma is more abrupt, akin to a promontory. The highest point in Istanbul is Çamlıca Hill (also on the Asian side), with an altitude of 288 meters (945 ft).

Istanbul is situated near the North Anatolian Fault on the boundary between the African and Eurasian plates. This fault zone, which runs from northern Anatolia to the Sea of Marmara, has been responsible for several deadly earthquakes throughout the city's history. Among the most devastating of these seismic events was the 1509 earthquake, which caused a tsunami that broke over the walls of the city, destroyed over 100 mosques, and killed more than 10,000 people. More recently, in 1999, an earthquake with its epicenter in nearby İzmit left 17,000 people dead, including 1,000 people in Istanbul's suburbs. Istanbulites remain concerned that an even more catastrophic seismic event may be in Istanbul's near future, as thousands of structures recently built to accommodate the city's rapidly increasing population may not have been constructed properly. Seismologists say the risk of a 7.6-magnitude earthquake striking Istanbul by 2030 is greater than sixty percent.

Istanbul has a Mediterranean climate according to the Köppen climate classification system, although its climate becomes more oceanic toward the north.

In summer the weather in Istanbul is hot and humid, with the temperature in July and August averaging 23 °C (73 °F).[76] Summers are relatively dry, but rainfall is significant during that season. Extreme heat, however, is uncommon, as temperatures rise above 32 °C (90 °F) on only five days per year on average. During winter it is cold, wet and often snowy, with the temperature in January and February averaging 4 °C (39 °F). Snowfalls tend to be heavy, but the snowcover and temperatures below the freezing point rarely last more than a few days. Spring and autumn are mild, but are unpredictable and often wet, and can range from chilly to warm, however the nights are chilly.

Istanbul has a persistently high humidity, which can exacerbate the moderate summer heat. The humidity is especially salient during the morning hours, when humidity generally reaches eighty percent and fog is very common. The city receives fog an average of 228 days each year, with the highest concentration of foggy days being in the winter months, although it usually dissipates by noontime. Thunderstorms are uncommon, occurring just 23 days each year, but they occur most frequently in the summer and early autumn months. Istanbul has an annual average of 124 days with significant precipitation, which together generate around 844 mm (33 in) of rain. The highest recorded temperature was 40.5 °C (105 °F) on 12 July 2000, and the lowest recorded temperature was −16.1 °C (3 °F) on 9 February 1927. Istanbul also tends to be a windy city, having an average wind speed of 18 km/h (11 mph). Due to the city's huge size, topography and maritime influences, Istanbul exhibits a multitude of distinct microclimates.

Istanbul has thirty-nine districts administered by the Istanbul Metropolitan Municipality (MMI). The district of Fatih, which includes the neighborhood and former district of Eminönü, is among the most central of these, residing on the historic peninsula south of the Golden Horn. The district corresponds to what was until the Ottoman conquest the whole of the city, across from which stood the Genoese citadel of Galata in the late Byzantine era. Those Genoese fortifications were largely demolished in the 19th century, leaving only the Galata Tower, to make way for northward expansion of the city. Galata is now a part of the Beyoğlu district, which forms Istanbul's commercial and entertainment center and includes İstiklal Avenue and Taksim Square.
Dolmabahçe Palace, the seat of government during the late Ottoman period, is located in Beşiktaş, just north of Beyoğlu, across from BJK İnönü Stadium, home to Turkey's oldest football club. The former village of Ortaköy is situated within Beşiktaş and provides its name to the Ortaköy Mosque, along the Bosphorus near the First Bosphorus Bridge. Lining the shores of the Bosphorus north of there are yalıs, luxurious chalet mansions originally built by 19th-century aristocrats and elites as summer homes. Today, some are homes within the city's most exclusive neighborhoods, including Bebek. Further inland, between the Bosphorus Bridge and the Fatih Sultan Mehmet (Second Bosphorus) Bridge, are Levent, Maslak, and Mecidiyeköy, Istanbul's primary economic centers. Officially part of the Beşiktaş and Şişli districts, they contain Istanbul's tallest buildings and the headquarters of Turkey's largest companies.

Like Beyoğlu, the districts of Üsküdar and Kadıköy on the Asian side were originally separate cities, Chrysopolis and Chalcedon, respectively. During the Ottoman period, they continued to remain outside the scope of urban Istanbul, serving as tranquil outposts with seaside yalıs and gardens. However, during the second half of the 20th century, the Asian side experienced massive urban growth, owning in part to the development of Bağdat Avenue into an upscale shopping hub similar to İstiklal Avenue on the European side. The fact that these areas were largely empty until the 1960s also provided the chance for developing better infrastructure and tidier urban planning when compared with most other residential areas in the city. While now officially parts of Istanbul, much of the Asian side of the Bosphorus, which accounts for one third of the city's population, functions as a suburb of the economic and commercial centers in European Istanbul.

As a result of Istanbul's exponential growth during the 20th century, a significant portion of the city's outskirts comprised gecekondus (a Turkish term meaning built overnight), referring to the illegally constructed squatter buildings run rampant outside the centers of the country's largest cities. At present, some gecekondu areas are being gradually demolished and replaced by modern mass-housing compounds.

Istanbul has two international airports: The larger one is the Atatürk International Airport located in the Yeşilköy district on the European side, about 24 kilometres (15 mi) west from the city centre. 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 Gökçen International Airport located in the Kurtköy 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 centre.

Sea transport is vital for Istanbul, as the city is practically surrounded by sea on all sides: the Sea of Marmara, the Golden Horn, the Bosphorus and the Black Sea. Many Istanbulites live on the Asian side of the city but work on the European side (or vice-versa) and the city's famous commuter ferries form the backbone of the daily transition between the two parts of the city – even more so than the two suspension bridges that span the Bosphorus. The commuter ferries, along with the high speed catamaran Seabus (Deniz Otobüsü), also form the main connection between the city and the Princes' Islands.

The first steam ferries appeared on the Bosphorus in 1837 and were operated by private sector companies. On 1 January 1851, the Şirket-i Hayriye (literally the Goodwill Company, as the Istanbul Ferry Company was originally called) was established by the Ottoman state. The Şirket-i Hayriye continued to operate the city's landmark commuter ferries until the early years of the Republican period, when they went under the direction of Türkiye Denizcilik İşletmeleri (Turkish State Maritime Lines). Since March 2006, Istanbul's traditional commuter ferries are operated by İstanbul Deniz Otobüsleri (Istanbul Sea Buses), which also operates the high speed catamaran Seabus.

İDO (İstanbul Deniz Otobüsleri – Istanbul Sea Buses) was established in 1987 and operates the high speed catamaran Seabuses that run between the European and Asian parts of Istanbul—and also connect the city with the Princes' Islands and other destinations in the Sea of Marmara. The Yenikapı High Speed Car Ferry Port on the European side, and the Pendik High Speed Car Ferry Port on the Asian side, are where the high speed catamaran "car ferries" are based. The car ferries that operate between Yenikapı (on the European side of Istanbul) and Bandırma reduce the driving time between Istanbul and İzmir and other major destinations on Turkey's Aegean coast by several hours; while those that operate between Yenikapı or Pendik (on the Asian side of Istanbul) and Yalova significantly reduce the driving time between Istanbul and Bursa or Antalya.

The port of Istanbul is the most important one in the country. The old port on the Golden Horn serves primarily for personal navigation, while Karaköy port in Galata is used by the large cruise liners. Regular services as well as cruises from both Karaköy and Eminönü exist to several port cities in the Mediterranean Sea and Black Sea. Istanbul's main cargo port is located in the Harem district on the Asian side of the city. Istanbul also has several marinas of varying size for personal navigation, the largest of which are the Ataköy Marina on the European side and Kalamış Marina on the Asian side.