"The visa waiver program (VWP) allows nationals from certain countries to enter the United States as temporary visitors (nonimmigrants) for business or pleasure without first obtaining a visa from a U.S. consulate abroad. Temporary visitors for business or pleasure from non-VWP countries must obtain a visa from Department of State (DOS) officers at a consular post abroad before coming to the United States. Concerns have been raised about the ability of terrorists to enter the United States under the VWP, because the VWP bypasses the first step by which foreign visitors are screened for admissibility to enter the United States. Nonetheless, there is interest in the VWP as a mechanism to promote tourism and commerce. In addition to increasing tourism, the inclusion of countries in the VWP may help foster positive relations between the United States and those countries, facilitate information sharing, and ease consular office workloads abroad. As of January 2014, 37 countries participate in the VWP. In FY2012, there were 19.1 million visitors who entered the United States under this program, constituting 40% of all overseas visitors."
CRS Report for Congress, RL32221