The city stands on the River Thames in the south-east of England, at the head of its 50-mile (80 km) estuary leading to the North Sea. London has been a major settlement for two millennia. Londinium was founded by the Romans. The City of London, London's ancient core and financial Centre − an area of just 1.12 square miles (2.9 km²) and colloquially known as the Square Mile − retains boundaries that closely follow its medieval limits. The adjacent City of Westminster has for centuries been the location of much of the national government. Thirty-one additional boroughs north and south of the river also comprise modern London. The London region is governed by the mayor of London and the London Assembly.

London is one of the world's most important global cities. It exerts a considerable impact upon the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism and transportation. It is one of the largest financial centers in the world and in 2019; London had the second highest number of ultra high-net-worth individuals in Europe, after Paris. And in 2020, London had the second-highest number of billionaires of any city in Europe, after Moscow. London's universities form the largest concentration of higher education institutes in Europe, and London is home to highly ranked institutions such as Imperial College London in natural and applied sciences, the London School of Economics in social sciences, and the comprehensive University College London. In 2012, London became the first city to have hosted three modern Summer Olympic Games.