The AC Transit District is the third-largest public bus system in California, serving a number of cities and unincorporated areas in Alameda and Contra Costa counties.

Three routes run across the San Francisco–Oakland Bay Bridge on a daily basis, connecting passengers in Alameda, Berkeley, Emeryville, and Oakland with the Salesforce Transit Center.

A late night-only bus operates overnight as a Transbay Replacement for BART service when BART is not running, connecting Albany, Berkeley, El Cerrito, Oakland, and Richmond with San Francisco's Central Business District along Market Street, as well as the Inner Mission and SoMa neighborhoods.

AC Transit's primary hubs include BART stations, major shopping centers, and points of interest, which are spread throughout the East Bay.

Service began in Fremont on November 12, 1974; in Newark on December 16, 1974; in Concord on September 8, 1975; in Pleasant Hill on December 8, 1975; in Moraga and Orinda on September 13, 1976; and in Antioch and Pittsburg (as Tri Delta Transit under contract to the Eastern Contra Costa Transit Authority) on June 5, 1977.

In 2004, the District began service on Line U across the Dumbarton Bridge, connecting Stanford University with ACE and BART trains in Fremont.

AC Transit had provided 24-hour service on many of its trunk lines prior to this date, except in the late 1990s due to budget limitations.

Bus stops are spaced 2/3-mile apart on average, running between Jack London Square (via 20th Street and Broadway) in Oakland and Contra Costa College in San Pablo, and buses receive signal priority at several intersections.

Tempo operates between the Uptown Transit Center and the San Leandro BART station via International Boulevard and East 14th Street.

It features 46 brand new platform stations (curbside and center-median) with dedicated bus lanes along the majority of the route.

At its inception, AC Transit purchased the mixed White, Mack, and GM "old-look" bus fleet from its predecessor, the Key System.

[22] By 1970, AC Transit was one of six agencies to participate in a "super bus project" coordinated by the National Transportation Center (Pittsburgh) to write a specification for a higher-capacity bus; once the specification had been written, two prototypes would be built and tested to select a winner for a large group procurement of 100 buses to keep per-unit costs low.

bus favorably, and the specification was released for bid in 1975;[26] AC Transit placed an order for 30 buses in 1976 and deliveries began from the AM General/M.A.N.

The Van Hool buses were assembled in Belgium and featured low floors and three doors (four doors on articulated models), which AC Transit touted as the key to bus rapid transit service between Berkeley and San Leandro along Shattuck, Telegraph, International Blvd, and East 14th Street.

[29] At the same time, AC Transit rolled out a revised "ribbon" livery featuring new colors (green and black), and a new logo.

[35] Later that year, in November 2013, new Gillig buses with a suburban seating configuration and Transbay branding were introduced into service.

[37] In 1969, AC Transit received a grant and converted bus #666 to steam power, which ran in revenue service between 1971 and 1972.

AC Transit took delivery of 12 additional third-generation fuel cell buses, based on the Van Hool A300L in 2011.

[46] AC Transit is funded with a mix of federal, state, and local government subsidies, as well as passenger fares.

)[47] In November 2004, voters approved Measure BB, which increased the parcel tax from US$24 to US$48 annually for 10 years beginning 1 July 2005, to help fund AC Transit services.

[49] In November 2008, voters approved Measure VV, which increased the parcel tax by US$48 annually for 10 years beginning 1 July 2009, to help fund AC Transit services.

[51] These buses can be distinguished by their all-green livery, padded "commuter" seats, and Wi-Fi logos near the front entrance door and inside the bus.