On the one-year anniversary of the implementation of the Electronic System for Travel Authorization (ESTA), U.S. Customs and Border Protection (CBP) issued a communication to remind U.S.-bound travelers from Visa Waiver Program (VWP) countries of their ESTA responsibilities. ESTA is an electronic travel authorization that all citizens from VWP countries must obtain prior to boarding a carrier to travel by air or sea to the United States under the VWP. VWP travelers are required to log onto the ESTA web site and complete an online application. ESTA has been mandatory since January 12, 2009, for all nationals of VWP countries travelling to the U.S. under the VWP. ESTA applications may be submitted at any time prior to travel, and, once approved, generally will be valid for up to two years or until the applicant's passport expires. Authorizations will be valid for multiple entries. Beginning January 20, 2010, CBP initiated a 60-day transition to enforce ESTA compliance for air carriers; as of March 20, 2010, the airlines are no longer supposed to allow VWP travelers to board a U.S.-bound plane without an approved ESTA application. Applicants may apply for ESTA online.