Because Venice is on a lagoon, the water plays a crucial role in transportation. The most popular way to approach Venice is by boat or train.
The closest commercial airport is Marco Polo Airport (ICAO: LIPZ, IATA: VCE), on the mainland near Mestre (a more typical Italian city, without Venice's unique structure). The Treviso Airport (ICAO: LIPH, IATA: TSF), located 25 km (16 mi) from Venice, is relatively smaller but becoming increasingly busy as the main destination for Ryanair, SkyEurope, and Transavia budget flights.
Both airports have bus connections with Venice (Piazzale Roma), Mestre, Padua and other towns. ATVO 'pullman'coaches (€10 return) run to and from Treviso to co-incide with flights. Marco Polo airport runs a shuttle bus --€3-- (or just turn left and walk 10 minutes under the awning) to the Alilaguna water-bus jetty, where €13 gets you a leisurely 75 minute boat trip to San Marco via Murano, Lido and the Arsenale. Or take the cheaper boat (€6,50) to Murano which takes only half an hour. Alternatively, you can travel in style (and much faster) by hiring one of the speedy water-taxis (30 mins) for about €100. All these tickets are now buyable online.
The San Nicolo Airport (ICAO: LIPV, IATA: ATC) is an airfield directly on the Lido. It handles only small aircraft, as the runway (grass) is about 1 km long, and does not have any scheduled flights, but might be of interest to private pilots (arrivals from Schengen states only) due to its convenience to the city (it is a short walk to the vaporetto landing).
Trains from the mainland run through Mestre to the Venezia - Santa Lucia train station on the west side of Venice (make sure you don't get confused with Venezia Mestre which is the last stop on the mainland!). From the station district, water buses (vaporetti) or water taxis can take you to hotels or other locations on the islands (or you can walk). Direct trains to Venice are available from many international destinations, including Munich, Budapest, Zagreb and Ljubljana. From Vienna (Wien) Trains can be arranged via the Austrian ÖBB train system.
Cars arrive on the far western edge of Venice, but remain parked at the entrance to the city (Piazzale Roma or Tronchetto - Europe's largest car park.) There are no roads past this point -- and never were, even before cars. Car parking is expensive here and the tailbacks can be quite large. An alternative is to use the car parks on the mainland (terra firma) and catch a vaporetto, train or bus into Venice. Park near the Mestre railway station, and catch a train to Venezia St.Lucia; there are many trains, it is very near (8-10 minutes) and quite cheap. (Don't bother searching for free parking near the train station - there are no free parking spots near.) Besides, Venezia St. Lucia is a good starting point to visit Venice. However drivers going to the Lido can use the car ferry from Tronchetto (vaporetto 17 - frequencies vary), right hand lane off the Ponte della Liberta into the city.
By rental car
Most of the major rental car companies have outlets at Piazzale Roma, at the edge of the city. These are on the ground floor of one of the major parking stations. When you are dropping off your car, you need to find street parking and then walk to the rental car outlet and hand in the keys. Do not park in the parking station! There is a vaporetto stop across the road from the parking station.
There is a direct bus between Marco Polo airport and the Piazzale Roma, on the west bank of Venice. Starts twice an hour, takes 20 minutes and costs €3. The Piazzale Roma bus station is well served by vaporetti and water-taxis ... and of course, you can walk everywhere. From Mestre, you can take a bus to Venezia- Piazzale Roma. the ticket is €1 but if you buy it in the bus it will cost €1.50. You can buy bus tickets from tobacconists and newsagencies. All of the city is connected to Venice by bus.
Ships arrive at the Stazione Marittima which is at the west end of the main islands, it is served by vaporetti and water taxis.