It is the most populous U.S. state, home to one out of eight people who live in the U.S., with a total of 38 million people, and it is the third largest state by area (after Alaska and Texas). California is bordered by Oregon to the north, Nevada to the east, Arizona to the southeast, and the Mexican state of Baja California to the south. It is home to the nation's second and fifth most populous census statistical areas (Greater Los Angeles Area and San Francisco Bay Area, respectively), and eight of the nation's 50 most populated cities (Los Angeles, San Diego, San Jose, San Francisco, Fresno, Sacramento, Long Beach, and Oakland). Sacramento is the state capital, and has been since 1854.

California's diverse geography ranges from the Pacific Coast in the west, to the Sierra Nevada in the east – from the Redwood–Douglas fir forests of the northwest, to the Mojave Desert areas in the southeast. The centre of the state is dominated by the Central Valley, a major agricultural area. California contains both the highest and lowest points in the contiguous United States (Mount Whitney and Death Valley), and has the 3rd longest coastline of all states (after Alaska and Florida).

Earthquakes are a common occurrence because of the state's location along the Pacific Ring of Fire: about 37,000 are recorded annually, but most are too small to feel.