Bay Area Rapid Transit (BART) is a rapid-transit commuter rail system serving the San Francisco Bay Area. The heavy-rail public transit system connects downtown San Francisco with suburbs in the East Bay and northern San Mateo County. BART operates five lines on 104 miles (167 km) of track with 43 stations in four counties. With average weekday ridership of 374,949 passengers, BART is the fifth busiest heavy rail rapid transit system in the United States.

BART is operated by the San Francisco Bay Area Rapid Transit District, a special-purpose transit district formed in 1957 covering San Francisco, Alameda, and Contra Costa counties. The name BART is an acronym and is pronounced as a word, not as individual letters. In some ways, BART is a successor to the Key System, which ran streetcars across the lower deck of the San Francisco Bay Bridge until 1958.