Frederick County is a county located in the northern part of the U.S. state of Maryland, bordering the southern border of Pennsylvania and the northeastern border of Virginia. As of the 2010 U.S. Census, the population was 233,385. It is a part of the Washington metropolitan area which is part of the greater Baltimore–Washington metropolitan area, yet is also sometimes recognized as part of Western Maryland. The county is home to Catoctin Mountain Park (encompassing the presidential retreat Camp David) and to the U.S. Army's Fort Detrick. The county seat is Frederick, which was 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.