The Alameda-Contra Costa Transit District is the third-largest public bus system in California, serving 13 cities and adjacent unincorporated areas in Alameda and Contra Costa counties. AC Transit has been serving the East Bay since 1960, taking over from the Key System and its predecessors, which carried passengers via buses, horse-drawn rail, electric streetcars, and ferries over the previous 100 years. AC Transit’s mission is to provide safe, convenient, courteous, and reliable transit service.

The District's service area:  Composed of approximately 1.5 million people who live in AC Transit’s 364 square mile service area.  Bus service is provided to these cities within Alameda & Contra Costa counties:  Alameda, Albany, Berkeley, El Cerrito, Emeryville, Fremont, Hayward, Newark, Oakland, Piedmont, Richmond, San Leandro, and San Pablo. 

The unincorporated area served are:  Ashland, Castro Valley, Cherryland, El Sobrante, Fairview, Kensington, Irvington, North Richmond, and San Lorenzo.

AC Transit also serves downtown San Francisco via the Bay Bridge, and Foster City and San Mateo via the San Mateo Bridge.  A new downtown San Francisco Transbay Transit Center is in progress.  The $4 billion Transbay Transit Center would replace the old bus terminal with a modern transit hub connecting 11 transportation systems, including AC Transit, BART, Caltrain, Golden Gate Transit, Greyhound, Muni, SamTrans, Amtrak and the future high-speed rail system between San Francisco and Los Angeles, according to the Transbay Joint Powers Authority.