London is the capital and largest city of both England and the United Kingdom, with a population of 8,866,180 in 2022. The wider metropolitan area is the largest in Western Europe, with a population of 14.9 million. London stands on the River Thames in southeast England, at the head of a 50-mile (80 km) estuary down to the North Sea, and has been a major settlement for nearly 2,000 years. Its ancient core and financial centre, the City of London, was founded by the Romans as Londinium and has retained its medieval boundaries. The City of Westminster, to the west of the City of London, has been the centuries-long host of the national government and parliament. London grew rapidly in the 19th century, becoming the world's largest city at the time. Since the 19th century, the name "London" has referred to the metropolis around the City of London, historically split between the counties of Middlesex, Essex, Surrey, Kent, and Hertfordshire, which since 1965 has largely comprised the administrative area of Greater London, governed by 33 local authorities and the Greater London Authority.

As one of the world's major global cities, London exerts a strong influence on world art, entertainment, fashion, commerce, finance, education, healthcare, media, science, technology, tourism, transport, and communications. Despite a post-Brexit exodus of stock listings from the London Stock Exchange, London remains a European economic powerhouse, and one of the world's major financial centres. It hosts Europe's largest concentration of higher education institutions, some of which are the highest-ranked academic institutions in the world: Imperial College London in natural and applied sciences, the London School of Economics in social sciences, and the comprehensive University College London. It is the most visited city in Europe and has the world's busiest city airport system. The London Underground is the world's oldest rapid transit system.

London's diverse cultures encompass over 300 languages. The 2023 population of Greater London of just under 10 million made it Europe's third-most populous city, accounting for 13.4% of the United Kingdom's population and over 16% of England's population. The Greater London Built-up Area is the fourth-most populous in Europe, with about 9.8 million inhabitants as of 2011. The London metropolitan area is the third-most populous in Europe, with about 14 million inhabitants as of 2016, making London a megacity.

Four World Heritage Sites are located in London: Kew Gardens; the Tower of London; the site featuring the Palace of Westminster, Church of St. Margaret, and Westminster Abbey; and the historic settlement in Greenwich where the Royal Observatory defines the prime meridian (0° longitude) and Greenwich Mean Time. Other landmarks include Buckingham Palace, the London Eye, Piccadilly Circus, St Paul's Cathedral, Tower Bridge, and Trafalgar Square. The city has the most museums, art galleries, libraries, and cultural venues in the UK, including the British Museum, National Gallery, Natural History Museum, Tate Modern, British Library, and numerous West End theatres. Important sporting events held in London include the FA Cup Final, the Wimbledon Tennis Championships, and the London Marathon. It became the first city to host three Summer Olympic Games upon hosting the 2012 Summer Olympics.