National Historic Landmarks (NHL) are nationally significant historic places designated by the U.S. Secretary of the Interior because they possess exceptional value or quality in illustrating or interpreting the heritage of the United States. It is the highest level of historic designation in the United States.
Today, just over 2,500 historic places across the country bear this national distinction. Working with citizens throughout the nation, the National Historic Landmarks Program draws upon the expertise of National Park Service staff, who guide the nomination process for new NHLs and provide assistance to existing NHLs. As with National Register listings, owners of NHLs are not legally restricted from making changes to their historic property, a number of incentives programs are available to encourage the preservation of these important resources.
Maryland contains seventy-three (73) National Historic Landmarks. Harford County has one National Historic Landmark, Sion Hill (HA-525, privately owned), located in Havre de Grace.