California is the most populous state of the United States. Located on the Pacific coast of North America, it is bordered by Oregon, Nevada and Arizona in the United States, and Baja California in Mexico. The state's four largest cities are Los Angeles, San Diego, San Jose and San Francisco. California is known for its diverse climate and ethnically diverse population. The state has 58 counties.

Inhabited by indigenous people for millennia, Alta California was first colonized by the Spanish Empire in 1769, and after Mexican independence in 1821, continued as part of Mexico. Following one brief week as the independent California Republic in 1846, and the conclusion of the Mexican-American war in 1848, California was annexed by the United States and was admitted to the Union as the thirty-first state on September 9, 1850.

California's diverse geography ranges from the sandy beaches of the Pacific coast to the rugged, snow-capped Sierra Nevada mountains in the east. The central portion of the state is dominated by the Central Valley, one of the most vital agricultural areas in the country. The Sierra Nevada contains Yosemite Valley, famous for its glacially-carved domes, and Sequoia National Park, home to the largest living organisms on Earth, the giant sequoia trees, and the highest point in the contiguous United States, Mount Whitney. The tallest living things on Earth, the ancient redwood trees, dot the coastline, mainly north of San Francisco. California is also home to the second lowest and hottest place in the Western Hemisphere, Death Valley. Bristlecone pines located in the White Mountains are the oldest known trees in the world; one has an age of 4,700 years.

The California Gold Rush, beginning in 1848, dramatically changed California with an influx of population and an economic boom, and San Francisco became a financial and cultural center. The early part of the 20th century was marked by Los Angeles becoming the center of the entertainment industry, in addition to the growth of a large tourism sector in the state. The Central Valley is home to California's important agricultural industry, the largest of any state. Other important industries have included the aerospace and oil industries. In recent decades, California has become a global leader in computers and information technology. If California were a country, its economy would rank among the ten largest in the world, and it would be the 35th largest country by population (behind Kenya).