From Foreign Policy:

For years citizens from 38 countries ranging from Andorra to New Zealand have been able to enter the United States without a visa. With terror risks growing, however, calls to get rid of the program are growing louder. Homeland Security Secretary Jeh Johnson defended the program Thursday against critics who say it raises the risk of terrorists from European or other nations in the program finding their way into the United States. Johnson said the program needed to be preserved, but acknowledged that it might need to be tweaked to ensure travelers from American allies could enter the United States relatively quickly without making it easier for terrorists to come in as well.