California's transportation system is complex and dynamic. Although known for its car culture and extensive network of freeways and roads, the state also has a vast array of rail, sea, and air transport. Several subway, light rail, and commuter rail networks are found in many of the state's largest population centers. In addition, with the state's location on the West Coast of the United States, several important ports in California handle freight shipments from the Pacific Rim and beyond. A number of airports are also spread out across the state, ranging from small general aviation airports to large international hubs like Los Angeles International Airport and San Francisco International Airport.

However, in a state with over 39 million people, rapid population expansion, and diverse terrain and weather, that system is under pressure to stay ahead of population growth and transportation needs.

California is known for its car culture; by the end of 2010, the California Department of Motor Vehicles had 23,799,513 driver's licenses and a total of 31,987,821 registered vehicles on file. The state's residents typically take to the roads for their commutes, errands, and vacations, giving California's cities a reputation for severe traffic congestion.

California's vast terrain is connected by an extensive system of freeways, expressways, and highways, all maintained by the California Department of Transportation (Caltrans) and patrolled by the California Highway Patrol (CHP), except for the numbered expressways in Santa Clara County which were built and maintained by the county itself. The main north–south arteries are U.S. Route 101 (US 101), which travels close to the coast from Downtown Los Angeles, across the Golden Gate Bridge, and to the Oregon state line, and Interstate 5 (I-5), which travels inland from the Mexico–United States border to the Oregon state line, effectively bisecting the entire state. In addition, a major north–south artery is State Route 99 (SR 99), which travels from near Bakersfield to near Red Bluff; SR 99 is largely parallel to I-5, and connects the Central Valley cities not connected by the Interstate.

Perhaps it is a testament to California's sheer size that although it has one of the most extensive freeway systems in the United States, it contains many of the largest cities in the United States not served by an Interstate Highway, including the two largest, Fresno (pop. 471,479) and Bakersfield (pop. 247,057). These cities, along with Modesto and Visalia, are passed by as I-5 veers west to avoid the congestion of the populated eastern side of the Central Valley as it connects Sacramento and Los Angeles.

The state's freeway network has expanded ever since the Arroyo Seco Parkway, the first freeway in the Western United States, connecting Los Angeles and Pasadena, opened in 1940. However, the state is not immune to freeway revolts, such as those that have prevented a freeway in San Francisco between the Golden Gate Bridge and the San Francisco–Oakland Bay Bridge, and an expansion of I-710 through South Pasadena.

Traffic signs, road surface markings, and traffic signals on all public streets and highways throughout the state are regulated by the California Vehicle Code and standardized in the California Manual on Uniform Traffic Control Devices.

Almost all California highways are non-toll roads, including several major non-toll bridges in Los Angeles, Sacramento, and San Diego. However, there are four toll roads in Southern California, and eight toll bridges in the San Francisco Bay Area (including the state's most famous highway bridge, the Golden Gate Bridge). These toll facilities, along with several high-occupancy toll lanes, use the statewide electronic toll collection system known as FasTrak.