A map of the locally accepted nine-county definition of the Bay Area. Also displayed are the five subregions of the Bay Area, which are divided along county lines except for the northwestern portion of Santa Clara county
The borders of the San Francisco Bay Area are not officially delineated, and the unique development patterns influenced by the region's topography, along with the presence of unusual commute patterns caused by the presence of three central cities and employment centers located in various suburban locales, has led to considerable disagreement between local and federal definitions of the area. Because of this, professor of geography at the University of California, Berkeley Richard Walker claimed that "no other U.S. city-region is as definitionally challenged [as the Bay Area]."
When the region began to rapidly develop during and immediately after World War II, local planners settled on a nine-county definition for the Bay Area, consisting of the counties that directly border the San Francisco, San Pablo, and Suisun estuaries: Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, Solano, and Sonoma counties. Today, this definition is accepted by most local governmental agencies including San Francisco Regional Water Quality Control Board, Bay Area Air Quality Management District, the San Francisco Bay Restoration Authority, the Metropolitan Transportation Commission, and the Association of Bay Area Governments, the latter two of which partner to deliver a Bay Area Census using the nine-county definition.
Various U.S. Federal government agencies use definitions that differ from their local counterparts' nine-county definition. For example, the Federal Communications Commission (FCC) which regulates broadcast, cable, and satellite transmissions, includes nearby Colusa, Lake and Mendocino counties in their "San Francisco-Oakland-San Jose" media market, but excludes eastern Solano county. On the other hand, the United States Office of Management and Budget, which designates Metropolitan Statistical Areas (MSAs) and Combined Statistical Areas (CSA) for populated regions across the country, has five MSAs which include, wholly or partially, areas within the nine-county definition, and one CSA which includes all nine counties plus neighboring San Benito, Santa Cruz and San Joaquin counties.
Among locals, the nine-county Bay Area can be further divided into five sub-regions: the East Bay, North Bay, South Bay, Peninsula, and the city of San Francisco. Although geographically located on the tip of the San Francisco Peninsula, the city of San Francisco is not considered part of the "Peninsula" subregion, but as a separate entity.
The "East Bay" is the densest region of the Bay Area outside of San Francisco and includes cities and towns in Alameda and Contra Costa counties, centered around Oakland. As one of the larger subregions, the East Bay includes a variety of enclaves, including the suburban Tri-Valley area and the highly urban western part of the subregion that runs alongside the bay. The "Peninsula" subregion includes the cities and towns on the San Francisco Peninsula, excluding the titular city of San Francisco. Its eastern half, which runs alongside the Bay, is highly populated while its less populated western coast traces the coastline of the Pacific Ocean and is known for its open space and hiking trails. Roughly coinciding with the borders of San Mateo county, it also includes the northwestern Santa Clara county cities of Palo Alto, Mountain View, and Los Altos. The "South Bay" includes all of the rest of the cities in Santa Clara county, centered around San Jose, the largest city in Northern California. It is roughly synonymous with Silicon Valley due to its high concentration of tech companies, although the industry also has a significant presence in the rest of the Bay Area. The "North Bay" includes Marin, Sonoma, Napa, and Solano counties, and is the largest and least populated subregion. The western counties of Marin and Sonoma are encased by the Pacific Ocean on the west and the bay on the east, and are characterized by its mountainous and woody terrain. Sonoma and Napa counties are known internationally for their grape vineyards and wineries, and Solano county to the east, centered around Vallejo, is the fastest growing region in the Bay Area.