If you work in California, any money that you earn for working there is subject to California tax. If you are not a resident of California you must file a California nonresident tax return and pay California tax. If you live in a state that has state income tax, depending on what state it is, you will either get a credit on your resident state tax return for part or all of the tax you pay to California, or you will get a credit on your California tax return for part or all of the tax you pay to the state that you live in.
