This designation, determined by the U.S. secretary of the interior, recognizes certain historic places for their exceptional value or quality in illustrating or interpreting the heritage of the United States. Less than 2,500 historical places bear this national distinction. Thirteen of these unique landmarks are located in the state of Delaware. The Division of Historical and Cultural Affairs owns two of these properties, the New Castle Court House in New Castle and the John Dickinson Plantation near Dover. Division staff interprets these nationally significant buildings through a variety of museum programs. In addition, the division owns several properties within the New Castle Historic District National Historic Landmark boundary, including the Sheriff's House, the Academy, the Arsenal and The Green itself.
National Park Service staff nominates new landmarks and provides assistance to existing landmarks. Theme studies, mandated by Congress, are the most effective way of identifying and nominating properties because they provide a comparative analysis of properties associated with a specific area of U.S. history as well as a national historic context. The National Park Service determines some nominations under cooperative agreements or contracts with other governmental entities such as the state of Delaware's State Historic Preservation Office, or private organizations.