London is the capital and largest city of England and the United Kingdom (UK). It is Britain's most populous and Europe's second most populous metropolitan area. A major settlement for two millennia, its history goes back to its founding by the Romans, who called it Londinium. London's core, the ancient City of London, or the 'square mile' financial district, largely retains its mediaeval boundaries. Since at least the 19th century, the name "London" has also referred to the metropolis developed around this core. In modern times, the bulk of this conurbation forms the London region and the Greater London administrative area,[note 1] with its own elected mayor and assembly.

London is a leading global city, the world's largest financial centre alongside New York, and has the largest city GDP in Europe. Central London is home to the headquarters of most of the UK's top 100 listed companies (the FTSE 100) and more than 100 of Europe's 500 largest. London's influence and strengths in the arts, education, entertainment, fashion, finance, healthcare, media, politics, professional services, sports, tourism, transport and culture in general all contribute to its global position. It is the most visited city in the world. London hosted the 1908 and 1948 Summer Olympics and will host the 2012 Summer Olympics. London contains four World Heritage Sites: the Tower of London; the Royal Botanic Gardens, Kew; the site comprising the Palace of Westminster, Westminster Abbey and St. Margaret's Church; and the historic settlement of Greenwich (in which the Royal Observatory marks the Greenwich Meridian (0° longitude) and GMT).

London has a diverse range of peoples, cultures and religions, and more than 300 languages are spoken within its boundaries. In July 2007 it had an official population of 7,556,900 within the boundaries of Greater London, making it the most populous municipality in the European Union. The Greater London Urban Area (the second largest in the EU) has a population of 8,278,251. while the metropolitan area (the largest in the EU) has an estimated total population of between 12 million and 14 million. The London Underground network, administered by Transport for London, is the oldest underground railway network in the world and the most extensive after the Shanghai Metro. London Heathrow Airport is the world's busiest airport by number of international passengers and the airspace is the busiest of any urban centre in the world.