Medieval London bridge (1209 - 1831) was not the first bridge built in that location. Previous bridges had been built by the Romans, the Saxons, and the Normans. When the bridge was built, King John allowed houses and shops to be built on it. This took up over half of the width of the bridge and caused many problems for people who attempted to cross it. These buildings were demolished in the middle of the 18th century.
The 19th Century London Bridge (1831 - 1971) was designed by John Rennie. In 1971, it was sold to an American investor named Robert McCulloch. The stones of the bridge were used as cladding on a steel bridge which spans a channel leading off Lake Havasu in Arizona, USA.
Modern London Bridge (1973 - present) was designed so that it would last. The current bridge is relatively plain compared to the other bridges over the Thames and is made of steel and pre-stressed concrete in order to enhance its durability.
Millennium Bridge (2000 - present) is a pedestrian-only bridge. It became internationally famous when pedestrians crossing it notice that it "wobbled." The wobble was fixed in 2002.
Tower Bridge (1894 - present) is definitely the most-photographed bridge of London. It may even be the most photographed bridge in the world. The outer edges are a suspension bridge, but the center is a bascule bridge. Bascule bridges open in the center to allow boats to pass through. The elevated parts in the center are pedestrian walkways.
Westminster Bridge (1862 - present) is the bridge closest to the Houses of Parliament, and is a popular place from which to photograph the Houses. As it is on the same end of the Houses of Parliament as the House of Commons, it is painted green to match the seats in that House. The current Westminster Bridge is the second bridge on that site (the first was opened in 1750).
