"Since the events of September 11, 2001, concerns have been raised about the ability of terrorists to enter the United States under the visa waiver program. 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. The VWP constitutes one of a few exceptions under the Immigration and Nationality Act (INA) in which foreign nationals are admitted into the United States without a valid visa. Under Department of Homeland Security (DHS) regulations, travelers who seek to enter the United States through the VWP are subject to the biometric requirements of the US-VISIT program."
CRS Report for Congress, RL32221
United States Diplomatic Mission to Italy: http://italy.usembassy.gov/english/