As described in Nolo's earlier update, New Restrictions on Who May Use Visa Waiver Program (VWP) Signed Into Law, Congress responded to concerns about global terrorism and the Paris bombings of 2015 by making changes to the Visa Waiver Program (VWP) in the 2016 Consolidated Appropriations Act (H.R. 2029). These changes included that non-citizens who have been present in the countries of Iraq, Syria, Iran, Sudan, or any other country that DHS designates as supporting terrorism or "of concern," at any time on or after March 1, 2011, are no longer eligible to participate in the VWP.
Since that announcement, DHS has, in fact, designated three other countries as "of concern." These include Libya, Somalia, and Yemen.
As before, the Secretary of Homeland Security may waive these restrictions on an individual, case-by-case basis if it's deemed to be in the U.S.'s law enforcement or national security interests. Categories of travelers potentially eligible for a waiver include those who traveled to these countries on behalf of international organizations, regional organizations, and sub-national governments on official duty; on behalf of a humanitarian NGO on official duty; or as journalists, for reporting purposes.
The restriction on Visa Waiver Program travel will not apply to dual nationals of these three countries.
DHS may still add more countries to this list.