London is the capital of both England and the United Kingdom, and the largest metropolitan area in the European Union. An important settlement for two millennia, London's history goes back to its founding by the Romans. Since its foundation, London has been part of many movements and phenomena throughout history, including the English Renaissance, the Industrial Revolution, and the Gothic Revival. The city's core, the ancient City of London, still retains its limited medieval boundaries; but since at least the 19th century, the name "London" has also referred to the whole metropolis that has developed around it. Today the bulk of this conurbation forms the London region of England and the Greater London administrative area, with its own elected mayor and assembly.

London is one of the the world's most important business, financial and cultural centres and its influence in politics, education, entertainment, media, fashion and the arts contributes to its status as a major global city. Central London is the headquarters of more than half of the UK's top 100 listed companies (the FTSE 100) and over 100 of Europe's 500 largest companies. The city is a major tourist destination for both domestic and overseas visitors, with annual expenditure by tourists of around £15 billion. London hosted the 1908 and 1948 Summer Olympic Games and will host the 2012 Summer Olympic Games. Samuel Johnson, author of the first complete dictionary, famously wrote about the city, citing “You find no man, at all intellectual, who is willing to leave London. No, Sir, when a man is tired of London, he is tired of life; for there is in London all that life can afford.”

Greater London contains four World Heritage Sites: the Tower of London; the historic settlement of Greenwich; the Royal Botanic Gardens, Kew; and the site comprising the Palace of Westminster, Westminster Abbey and St. Margaret's Church.

London's population draws from a wide range of peoples, cultures, and religions, and over 300 languages are spoken within the city. As of 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, with a population more than double that of its nearest rival. As of 2001, the Greater London Urban Area had a population of 8,278,251 and the metropolitan area is estimated to have a total population of just under 14 million, the largest metropolitan area in the EU. The public transport network, administered by Transport for London, is one of the most extensive in the world, Heathrow Airport is the busiest airport in the world by international passenger traffic and the air space is the busiest of any city in the world.

Greater London covers an area of 607 square miles (1,570 km2). Its primary geographical feature is the Thames, a navigable river which crosses the city from the south-west to the east. The Thames Valley is a floodplain surrounded by gently rolling hills including Parliament Hill, Addington Hills, and Primrose Hill. The Thames was once a much broader, shallower river with extensive marshlands; at high tide, its shores reached five times their present width. Since the Victorian era it has been extensively embanked, and many of its London tributaries now flow underground. The Thames is a tidal river, and London is vulnerable to flooding. The threat has increased over time due to a slow but continuous rise in high water level by the slow 'tilting' of Britain (up in the north and down in the south) caused by post-glacial rebound. In 1974, a decade of work began on the construction of the Thames Barrier across the Thames at Woolwich to deal with this threat. While the barrier is expected to function as designed until roughly 2030, concepts for its future enlargement or redesign are already being discussed.

London has a temperate marine climate, like much of the British Isles, so the city rarely sees extremely high or low temperatures. Summers are warm with average high temperatures of 23 °C (73 °F) and lows of 14 °C (57 °F), however, temperatures could exceed 25 °C (77 °F) on many days. Winters in London are chilly, but rarely below freezing with daytime temperatures around 2 - 8 °C (36 - 46 °F), while spring has mild days and cool evenings.

London has regular but generally light precipitation throughout the year, with average precipitation of 583.6 millimetres (22.98 in) every year. Snow is relatively uncommon, particularly because heat from the urban area can make London up to 5 °C (9 °F) hotter than the surrounding areas in winter. Some snowfall, however, is usually seen up to a few times a year. London is in USDA Hardiness zone 9, and AHS Heat Zone 2.

In the second half of the 19th century and the first half of the 20th, London was noted for its dense fogs and smogs. Following the deadly Great Smog of 1952, the Clean Air Act 1956 was passed, leading to the decline of such severe pollution in the capital.

London is too diverse to be characterised by any particular architectural style, having accumulated its buildings over a long period of time and drawn on a wide range of influences. It is, however, mainly brick built, most commonly the yellow London stock brick or a warm orange-red variety, often decorated with carvings and white plaster mouldings. Many grand houses and public buildings (such as the National Gallery) are constructed from Portland stone. Some areas of the city, particularly those just west of the centre, are characterised by white stucco or whitewashed buildings. Few structures pre-date the Great Fire of 1666, except for a few trace Roman remains, the Tower of London and a few scattered Tudor survivors in the City. The disused (but soon to be rejuvenated) 1939 Battersea Power Station by the river in the south-west is a local landmark, while some railway termini are excellent examples of Victorian architecture, most notably St Pancras and Paddington (at least internally).
The O2, one of the largest dome structures in the world.

The density of London varies, with high employment density in the central area, high residential densities in inner London and lower densities in the suburbs. In the dense areas, most of the concentration is achieved with medium- and high-rise buildings. London's skyscrapers such as the notable "Gherkin", Tower 42, the Broadgate Tower and One Canada Square are usually found in the two financial districts, the City of London and Canary Wharf. Other notable modern buildings include City Hall in Southwark with its distinctive oval shape, the British Library in Somers Town/Kings Cross, What was formerly the Millennium Dome, located by the Thames to the east of Canary Wharf, is now used as an entertainment venue known as The O2.

The development of tall buildings has been encouraged in the London Plan, which will lead to the erection of many new skyscrapers over the next decade, particularly in the City of London and Canary Wharf. The 72-storey, 1,017 feet (310 m) "Shard London Bridge" by London Bridge station, the 945 feet (288 m) Bishopsgate Tower and many other skyscrapers over 500 feet (150 m) are either proposed or approved and could transform the city's skyline. As of July 2008, there are 426 high-rise buildings (between 23 m to 150 m/75 ft to 491 ft) under construction, approved for construction, and proposed for construction in London.

A great many monuments pay homage to people and events in the city. The Monument in the City of London provides views of the surrounding area while commemorating the Great Fire of London, which originated nearby. Marble Arch and Wellington Arch, at the north and south ends of Park Lane respectively, have royal connections, as do the Albert Memorial and Royal Albert Hall in Kensington. Nelson's Column is a nationally recognised monument in Trafalgar Square, one of the focal points of the centre.