U.S. Customs and Border Protection requires all vehicles imported into the country to meet the various regulations specified by both the Environmental Protection Agency and the Department of Transportation. The actual regulations vary depending on the type of vehicle being imported but not by country of origin.
When importing a vehicle into the United States, the owner must show valid proof of ownership and also prove that it meets both the EPA and DOT regulations. If the vehicle has a DOT sticker on the inside of the driver's door and an EPA sticker on the engine that shows it was manufactured to U.S. specifications, this is all the proof that is needed. However, vehicles that do not have these stickers require a separate manufacturer's letter or certificate to prove that it meets all the required regulations.
Vehicles older than 25 years do not have to meet the EPA and DOT regulations, but the owner is still obligated to fill out the DOT and EPA forms and present them to a Customs agent at the time of import. Vehicles that are less than 25 years old do not have to meet the regulations at the time of import, but they must then be brought into compliance with the regulations when exported or sold.