Travelers who qualify for visa free travel under the Visa Waiver Program may transit the United States. Prior to boarding the air or sea carrier you are required to obtain travel authorization under ESTA.
If transiting the United States to a destination in Canada, Mexico or the adjacent islands, you may re-enter the United States on the return journey using any mode of transport, as long as the total visit, including both periods of time spent in transit and in Canada, Mexico or the adjacent islands, does not exceed 90 days. If transiting to a destination outside of Canada, Mexico or the adjacent islands, the return journey must be on a participating carrier, but need not be within 90 days, as you will be required to make a new application for admission. If you are transiting the United States to take up residence in Mexico, Canada, Bermuda or the Caribbean Islands you must be legal permanent residents of these areas.