Frederick County is included in the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area. As a result, the county has experienced a rapid population increase in recent years. The county is sometimes associated with Western Maryland, depending on the definition used. It borders the southern border of Pennsylvania and the northeastern border of Virginia.
The county is home to Catoctin Mountain Park (encompassing the presidential retreat Camp David) and to the U.S. Army's Fort Detrick. It has also been the home to several celebrated historical figures like Francis Scott Key, Thomas Johnson, Roger B. Taney, and Barbara Fritchie.
In 1776, Frederick County was divided into three parts. The westernmost portion became Washington County, named after George Washington, the southernmost portion became Montgomery County, named after another Revolutionary War general, Richard Montgomery. The northern portion remained Frederick County.
In 1837 a part of Frederick County was combined with a part of Baltimore County to form Carroll County which is east of current day Frederick County.
The county has a number of properties on the National Register of Historic Places.