Today, “London” usually refers to the London region of England, which is coterminous with Greater London. At the heart of the conurbation is the small, ancient City of London which was historically the entirety of the city. Londoners generally refer to the City of London simply as “the City” or the “Square Mile.” London’s metropolitan area grew considerably during the Victorian era and again during the Interwar period with expansion halted in the 1940s by World War II and Green Belt legislation and has been largely static since.

Greater London covers an area of 609 square miles. Its primary geographical feature is the Thames, a navigable river which crosses the city from the southwest to the east. The Thames Valley is a floodplain surrounded by gently rolling hills such as Parliament Hill and Primrose Hill. These hills presented no significant obstacle to the growth of London from its origins as a port on the north side of the river, and therefore London is roughly circular.

The Thames was once a much broader, shallower river with extensive marshlands. 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. The Thames Barrier was constructed across the Thames at Woolwich in the 1970s to deal with this threat, but a more substantial barrier further downstream may be necessary in the near future.