The United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Sites are places of importance to cultural or natural heritage as described in the UNESCO World Heritage Convention, established in 1972. The United States of America ratified the convention on December 7, 1972, making its historical sites eligible for inclusion on the list.
The first sites in the United States added to the list were Mesa Verde National Park and Yellowstone National Park, both at the Second Session of the World Heritage Committee, held in Washington, D.C. from September 5–8, 1978. In total, 21 sites have been included, the most recent being Papahānaumokuākea in 2010. The twenty-one sites are located in eighteen different states and two territories. California, Hawaii, Montana, and New Mexico each contain multiple sites, while two sites (Waterton-Glacier International Peace Park and Kluane / Wrangell – St. Elias / Glacier Bay / Tatshenshini-Alsek) are also in Canada.