The city stretches across numerous small islands in the marshy Venetian Lagoon along the Adriatic Sea in northeast Italy. The saltwater lagoon stretches along the shoreline between the mouths of the Po (south) and the < Piave (north) Rivers. The population estimate of 272,000 inhabitants includes the population of the whole Comune of Venezia; the historic city of Venice (Centro storico) inhabitants are around 62,000, while approximately 176,000 people live in Terraferma (literally firm land, it means the extra-lagoon areas), mostly in the large frazione of Mestre and 31,000 live in other islands of the lagoon. 

The Venetian Republic was a major sea power and a staging area for the Crusades, as well as a very important centre of commerce (especially the spice trade and art in the Renaissance Venice is world-famous for its canals. It is built on an archipelago of 118 islands formed by about 150 canals in a shallow lagoon. The islands on which the city is built are connected by about 400 bridges. In the old center, the canals serve the function of roads, and every form of transport is on water or on foot. In the 19th century a causeway to the mainland brought a railway station to Venice, and an automobile causeway and parking lot was added in the 20th century. Beyond these land entrances at the northern edge of the city, transportation within the city remains, as it was in centuries past, entirely on water or on foot. Venice is Europe's largest urban carfree area, unique in Europe in remaining a sizable functioning city in the 21st century entirely without motorcars or trucks.

The classical Venetian boat is the gondola, although it is now mostly used for tourists, or for weddings, funerals, or other ceremonies. Most Venetians now travel by motorised waterbuses ("vaporetti") which ply regular routes along the major canals and between the city's islands. The city also has many private boats. The only gondolas still in common use by Venetians are the traghetti, foot passenger ferries crossing the Grand Canal at certain points without bridges. Venice is served by the newly rebuilt Marco Polo International Airport, or Aeroporto di Venezia Marco Polo, named in honor of its famous citizen. The airport is on the mainland and was rebuilt away from the coast so that visitors now need to get a bus to the pier, from which a water taxi or Alilaguna waterbus can be used. 

The buildings of Venice are constructed on closely spaced wood piles(under water, in the absence of oxygen, wood does not decay which penetrate alternating layers of clay and sand. Wood for piles was cut in the most western part of todays Slovenia, resulting in the barren land called Karst. Most of these piles are still intact after centuries of submersion. The foundations rest on the piles, and buildings of brick or stone sit above these footings. The buildings are often threatened by flood tides pushing in from the Adriatic between autumn and early spring. Six hundred years ago, Venetians protected themselves from land-based attacks by diverting all the major rivers flowing into the lagoon and thus preventing sediment from filling the area around the city. This created an ever-deeper lagoon environment. During the 20th century, when manyartesian wells were sunk into the periphery of the lagoon to draw water for local industry, Venice began to subside. It was realised that extraction of the aquifer was the cause. This sinking process has slowed markedly since artesian wells were banned in the 1960s. However, the city is still threatened by more frequent low-level floods (so-called Acqua alta, "high water") that creep to a height of several centimeters over its quays, regularly following certain tides. In many old houses the former staircases used by people to unload goods are now flooded, rendering the former ground floor uninhabitable. Thus, many Venetians resorted to moving up to the upper floors and continuing with their lives. Some recent studies have suggested that the city is no longer sinking, but this is not yet certain; therefore, a state of alert has not been revoked. In May 2003 the Italian Prime Minister Silvio Berlusconi inaugurated the MOSE project (Modulo Sperimentale Elettromeccanico), an experimental model for evaluating the performance of inflatable gates; the idea is to lay a series of 79 inflatable pontoons across the sea bed at the three entrances to the lagoon. When tides are predicted to rise above 110 centimetres, the pontoons will be filled with air and block the incoming water from the Adriatic sea. This challenging engineering work is due to be completed by 2011. Some experts say that the best way to protect Venice is to physically lift the City to a greater height above sea level - by pumping water into the soil underneath the city. This way, some hope, it could rise above sea levels, protecting it for hundreds of years, and eventually the MOSE project may not be necessary (it will, controversially, alter the tidal patterns in the lagoon, damaging some wildlife). A further point about the "lifting" system would be that it would be permanent - the MOSE Project is, by its very nature, a temporary system: it is expected to protect Venice for "only" 100 years.