San Francisco International Airport
County: San Mateo County
San Francisco is a major international airport with direct service to major cities around the world. It is served by major US and foreign airlines. The airport has three major domestic terminals (1, 2, and 3) and one international terminal. (airline information)
An automated train system called AirTrain provides free transportation between the terminals, the BART station at the airport, some parking garages, and the Rental Car Center. AirTrain runs every few minutes 24 hours a day.
BART trains stop at the San Francisco International Airport Station, which is located by the International Terminal. The BART system serves Downtown San Francisco and the East Bay and trains generally operate every 15 to 20 minutes 7 days a week.
SFO employees are eligible for a fare discount on BART. 
Caltrain, which serves San Jose, stops a mile away from the airport at Millbrae. SamTrans bus route SFO provides direct connection between Millbrae Station and the SFO terminals 7 days a week. Also, BART provides connection from Millbrae Station to SFO (direct ride on BART is available only on nights and weekends).
BART provides connection between Amtrak trains at Richmond Station and SFO.
Bus - SamTrans
SamTrans provides low cost local and express bus service.
Line 292/397 is the cheapest form of transportation to/from San Francisco. SamTrans buses that serve the terminal run on the lower level roadway serving the same stops. See the headsign for bus destination.
Airporters provide long distance bus service. Advance reservation is required for some companies.
Bicyclist are prohibited from riding on the main roadway at the airport, but are allowed on McDonnell Road, North Link Road, and South Link Road.
Bicycle parking is available on the ground level by the International Terminal and inside Domestic Garage (valet parking) level 4, section C. Bicycles can be parked for up to 14 days by registering with an on-duty security guard or valet agent. Registration is not needed for parking less than 24 hours.