California State Highway 99 is a 415 mile (668 km) highway that traverses California's Central Valley from the north at Red Bluff to the south near Bakersfield. The majority of its length is built to freeway standards, though it is at times a two-lane rural highway or a divided highway.

CA-99 was originally part of U.S. Highway 99 which was decommissioned in 1968 after the completion of Interstate 5. Since the remnant did not cross state lines, it was not allowed to keep its federal highway status. Many of the older highway signs in the southbound lanes still display a control city of Los Angeles, even though CA-99 no longer runs through that city. From the north, 99 runs generally parallel to, and to the east of Interstate 5. It begins by splitting off from I-5 at Red Bluff, and serves as a two-lane highway, running through Chico and Yuba City to the state capital, Sacramento. There, it is promoted to a freeway and meets I_5 briefly before diverging from it again. The two freeways run somewhat close to one another for about 90 miles, but after passing through Stockton, they begin to diverge more and more.

Traveling southbound form Stockton, CA-99 passes through the cities of the San Joaquin Valley, while I_5 is relegated to less densely populated areas. Travelers on their way to Los Angeles from Stockton and Sacramento, or from Los Angeles to the Bay Area would be well advised to take I_5 and avoid the traffic jams of CA_99. CA_99 continues through Modesto, Merced, Fresno, Visalia, Tulare and Bakersfield. A few miles before the Tehachapi Mountains and Grapevine Hill, the road made famous by Commander Cody and The Lost Planet Airmen in their song, "Hot Rod Lincoln," CA-99 meets up with I-5 again and comes to an end in Wheeler Ridge.