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. Notable persons from here include Francis Scott Key, Chris Rose, Zach Taylor, Matt Bennett, Thomas Johnson, Roger B. Taney, and Barbara Fritchie.
The namesake of Frederick County and its county seat is unknown, but it probably was either Frederick, Prince of Wales, or Frederick Calvert, 6th Baron Baltimore.
Frederick County was created in 1748 by the Province of Maryland from parts of Prince George's County and Baltimore County.
In 1776 following Independence, 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.