Frederick County, Maryland Frederick County, Maryland Frederick County is located in the northern part of the U.S. state of Maryland. As of the 2010 U.S. Census, the population was 240,336. The county seat is Frederick. Frederick County is included in the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area. Like other outlying sections of the Washington metropolitan area, Frederick County has experienced a rapid population increase in recent years. 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
Montgomery County, Maryland Montgomery County, Maryland Montgomery County is the most populous county in the U.S. state of Maryland, located adjacent to Washington, D.C. As of the 2010 census, the county's population was 971,777, increasing by 9.0% to an estimated 1,058,810 in 2017. The county seat and largest municipality is Rockville, although the census-designated place of Germantown is the most populous place. Montgomery County is included in the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area, which in turn forms part of the Baltimore–Washington Combined Statistical Area. Most of the county's residents live in unincorporated locales, of which the most built up are Silver Spring and