Tehran (/tɛəˈræn, -ˈrɑːn, ˌteɪə-/; Persian: تهران‎ Tehrân [tehˈɾɒːn]is the capital of Iran and Tehran Province. With a population of around 8.7 million in the city and 15 million in the larger metropolitan area of Greater Tehran, Tehran is the most populous city in Iran and Western Asia, and has the second-largest metropolitan area in the Middle East. It is ranked 24th in the world by the population of its metropolitan area.

In the Classical era, part of the territory of present-day Tehran was occupied by Rhages, a prominent Median city.^[8] It was subject to destruction through the medieval Arab, Turkic, and Mongol invasions. Its modern-day inheritor remains as an urban area absorbed into the metropolitan area of Greater Tehran.

Tehran was first chosen as the capital of Iran by Agha Mohammad Khan of the Qajar dynasty in 1796, in order to remain within close reach of Iran's territories in the Caucasus, before being separated from Iran as a result of the Russo-Iranian Wars, and to avoid the vying factions of the previously ruling Iranian dynasties. The capital has been moved several times throughout the history, and Tehran is the 32nd national capital of Iran. Large scale demolition and rebuilding began in the 1920s, and Tehran has been a destination for mass migrations from all over Iran since the 20th century.

Tehran is home to many historical collections, including the royal complexes of Golestan, Sa'dabad, and Niavaran, where the two last dynasties of the former Imperial State of Iran were seated. Tehran's most famous landmarks include the Azadi Tower, a memorial built under the reign of Mohammad Reza Shah of the Pahlavi dynasty in 1971 to mark the 2,500th year of the foundation of the Imperial State of Iran, and the Milad Tower, the world's sixth-tallest self-supporting tower which was completed in 2007. The Tabiat Bridge, a newly-built landmark, was completed in 2014.

The majority of the population of Tehran are Persian-speaking people, and roughly 99% of the population understand and speak Persian, but there are large populations of other ethno-linguistic groups who live in Tehran and speak Persian as a second language.

Tehran has an international airport (Imam Khomeini Airport), a domestic airport (Mehrabad Airport), a central railway station, the rapid transit system of Tehran Metro, a bus rapid transit system, trolleybuses, and a large network of highways.

There have been plans to relocate Iran's capital from Tehran to another area, due mainly to air pollution and the city's exposure to earthquakes. To date, no definitive plans have been approved. A 2016 survey of 230 cities by consultant Mercer ranked Tehran 203rd for quality of life. According to the Global Destinations Cities Index in 2016, Tehran is among the top ten fastest growing destinations.

October 6 is marked as Tehran Day based on a 2016 decision by members of the City Council, celebrating the day when the city was officially chosen as the capital of Iran by the Qajar dynasty back in 1907.

The growing social awareness of civil rights resulted in the Constitutional Revolution and the first constitution of Iran in 1906. On June 2, 1907, the parliament passed a law on local governance known as the Baladie (municipal law), providing a detailed outline on issues such as the role of councils within the city, the members' qualifications, the election process, and the requirements to be entitled to vote. The then Qajar monarch Mohammad Ali Shah abolished the constitution and bombarded the parliament with the help of the Russian-controlled Cossack Brigade on June 23, 1908. That followed the capture of the city by the revolutionary forces of Ali-Qoli Khan (Sardar Asad II) and Mohammad Vali Khan (Sepahsalar e Tonekaboni) on July 13, 1909. As a result, the monarch was exiled and replaced with his son Ahmad, and the parliament was re-established.

After World War I, the constituent assembly elected Reza Shah of the Pahlavi dynasty as the new monarch, who immediately suspended the Baladie law of 1907, replacing the decentralized and autonomous city councils with centralist approaches of governance and planning.

From the 1920s to the 1930s, under the rule of Reza Shah, the city was essentially rebuilt from scratch. That followed a systematic demolition of several old buildings, including parts of the Golestan Palace, Tekye Dowlat, and Tupkhane Square, which were replaced with modern buildings influenced by classical Iranian architecture, particularly the building of the National Bank, the Police Headquarters, the Telegraph Office, and the Military Academy.

The changes in urban fabric started with the street-widening act of 1933, which served as a framework for changes in all other cities. The Grand Bazaar was divided in half and many historic buildings were demolished to be replaced with wide straight avenues.^[27] As a result, the traditional texture of the city was replaced with intersecting cruciform streets that created large roundabouts, located on major public spaces such as the bazaar.

As an attempt to create a network for easy transportation within the city, the old citadel and city walls were demolished in 1937, replaced by wide streets cutting through the urban fabric. The new city map of Tehran in 1937 was heavily influenced by modernist planning patterns of zoning and gridiron networks.

During World War II, Soviet and British troops entered the city. In 1943, Tehran was the site of the Tehran Conference, attended by U.S. President Franklin D. Roosevelt, Soviet Premier Joseph Stalin, and British Prime Minister Winston Churchill.

The majority of Tehranis are officially Twelver Shia Muslims, which has also been the state religion since the 16th-century Safavid conversion. Other religious communities in the city include followers of the Sunni and Mystic branches of Islam, various Christian denominations, Judaism, Zoroastrianism, and the Bahá'í Faith.

There are many religious centers scattered around the city, from old to newly-built centers, including mosques, churches, synagogues, and Zoroastrian fire temples. The city also has a very small third-generation Indian Sikh community that has a local gurdwara that was visited by the Indian Prime Minister in 2012

Tehran, as one of the main tourist destinations in Iran, has a wealth of cultural attractions. It is home to royal complexes of Golestan, Saadabad and Niavaran, which were built under the reign of the country's last two monarchies.

There are several historic, artistic and scientific museums in Tehran, including the National Museum, the Malek Museum, the Cinema Museum at Ferdows Garden, the Abgineh Museum, Museum of the Qasr Prison, the Carpet Museum, the Reverse Glass Painting Museum (vitray art), and the Safir Office Machines Museum. There is also the Museum of Contemporary Art, which hosts works of famous artists such as Van Gogh, Pablo Picasso, and Andy Warhol.

The Iranian Imperial Crown Jewels, one of the largest jewel collections in the world, are also on display at Tehran's National Jewelry Museum.

A number of cultural and trade exhibitions take place in Tehran, which are mainly operated by the country's International Exhibitions Company. Tehran's annual International Book Fair is known to the international publishing world as one of the most important publishing events in Asia.

Buses have served the city since the 1920s. Tehran's transport system includes conventional buses, trolleybuses, and bus rapid transit(BRT). The city's four major bus stations include the South Terminal, the East Terminal, the West Terminal, and the northcentral Beyhaghi Terminal.

The trolleybus system was opened in 1992, using a fleet of 65 articulated trolleybuses built by Czechia's Škoda.^[62] This was the first trolleybus system in Iran. In 2005, trolleybuses were operating on five routes, all starting at Imam Hossein Square.^[63] Two routes running northeastwards operate almost entirely in a segregated busway located in the middle of the wide carriageway along Damavand Street, stopping only at purpose-built stops located about every 500 metres along the routes, effectively making these routes trolleybus-BRT (but they are not called such). The other three trolleybus routes run south and operate in mixed-traffic. Both route sections are served by limited-stop services and local (making all stops) services. A 3.2-kilometer extension from Shoosh Square to Rah Ahan Square was opened in March 2010.

Tehran's bus rapid transit (BRT) was officially inaugurated in 2008. It has 10 lines with some 215 stations in different areas of the city. As of 2011, the BRT system had a network of 100 kilometres (62 miles), transporting 1.8 million passengers on a daily basis. The city has also developed a bicycle sharing system that includes 12 hubs in one of Tehran's districts.

Tehran has a central railway station that connects services round the clock to various cities in the country, along with a Tehran–Europe train line also running.

The feasibility study and conceptual planning of the construction of Tehran's subway system were started in the 1970s. The first two of the eight projected metro lines were opened in 2001.

There are over 2,100 parks within the metropolis of Tehran with one of the oldest being Jamshidie Park, which was first established as a private garden for Qajar prince Jamshid Davallu, and was then dedicated to the last empress of Iran, Farah Pahlavi. The total green space within Tehran stretches over 12,600 hectares, covering over 20 percent of the city's area. The Parks and Green Spaces Organization of Tehran was established in 1960, and is responsible for the protection of the urban nature present in the city.

Tehran's Birds Garden is the largest bird park of Iran. There is also a zoo located on the Tehran–Karaj Expressway, housing over 290 species within an area of about five hectares