There are several operators of bus services for San Francisco Airport.
SamTrans runs scheduled services between downtown San Francisco and SFO.
The SamTrans KX Express serves the Transbay Terminal in downtown San Francisco and San Francisco Airport. It drops off/picks up on the Arrivals level of Terminal 1 and Courtyard A and G of the International Terminal.
SamTrans buses to SFO depart the Transbay Terminal, downtown San Francisco from approximately 5.30 am until just before 10.30 pm weekdays, 6.15 am until 9.15 pm weekends.
SamTrans buses to downtown San Francisco depart SFO from 6.10 am until just after 10.30 pm weekdays, from just before 7.30 am until 9.30 pm weekends.
The one-way Samtrans KX Express fare between downtown San Francisco and SFO is $5.
SamTrans information is available on 800 660-4287 or online.
Airporter buses serve many Bay Area destinations to/from SFO.
They depart from the Arrivals level of domestic Terminals 1 and 2 and the International Terminal. They drop off on the Departures levels of the terminals.
Companies that provide Airporter services include:
Evans Airport Service - (707) 255-1559
Greyhound Lines - 800 231-2222
Marin Airporter - (415) 461-4222
Monterey Airbus - (831) 373-7777
Santa Cruz Airport Flyer - (831) 423-5937
Sonoma County Airport Express - 800 327-2042