Driver's Licenses
If you are temporarily visiting California, you can use your out-of-state or foreign driver's license for as long as it's valid where it was issued — you do not need to register with the DMV or do anything beyond making sure your license is valid and current. In most cases, (e.g. Western European countries, Australia, most of SE. Asia, etc.) foreigners do not really need one of those additional paper International Driver's License as well, but if your license looks exotic, it would probably help to have one just in case (this is the license issued by the relevant Automobile Association or Government body in your country that certifies in several languages that you have a license in your own country. You must also have your normal license with you if you use the International License).
