Contra Costa County (Contra Costa, Spanish for "Opposite Coast") is located in the eastern San Francisco Bay Area of California. As of the 2020 census, the population was 1,165,927. The county seat is Martinez. It occupies the northern portion of the East Bay region of the San Francisco Bay Area and is primarily suburban, and was one of the original 27 counties established in California in 1850.

Contra Costa County is famed for its mild Mediterranean climate, its wealth of sunny open spaces served by biking, hiking, and equestrian trails, and its easy access to San Francisco via the Bay Area Rapid Transit (BART) and Interstate Highways.

Comprised of 30 towns and cities, Contra Costa County is the ninth most populous county in the state, which include: Alamo, Antioch, Bethel Island, Brentwood, Byron Canyon, Clayton, Concord, Crockett, Danville, Diablo, Discovery Bay, El Cerrito, El Sobrante, Hercules, Knightsen, Lafayette, Martinez, Moraga, Oakley, Orinda, Pinole, Pittsburg, Pleasant Hill, Port Costa, Richmond, Rodeo, San Pablo, San Ramon and Walnut Creek.

The most notable landmark in the county is Mount Diablo, rising 3,849 feet at the northern end of the Diablo Range. It is the centerpiece of Mt. Diablo State Park, one of many recreation areas residents and visitors can enjoy. The East Bay Regional Park District is comprised of more than 100,000 acres within 65 parks, including more than 1,200 miles of trails.

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