Middlesex,county, northeasternMassachusetts, U.S., west and northwest ofBoston and bordered on the north byNew Hampshire. The county consists of an upland region drained by theMerrimack,Nashua, Assabet,Concord,Sudbury, and Shawsheen rivers. Other waterways include Whitehall and Cambridge reservoirs, Lake Cochituate, and historicWalden Pond. Parklands include more than 20 state and federal sites, notably Townsend and Willard Brook state forests, Hopkinton and Cochituate state parks, and Minute Man National Historical Park.

Middlesex was created in May 1643 as one of Massachusetts’ three original counties and was named for Middlesex, England. The county seat isCambridge, the home ofHarvard University (founded 1636) and theMassachusetts Institute of Technology (1861). Other institutions ofhigher education includeBoston College (1863) in Chestnut Hill–Newton,Tufts University (1852) inMedford, andBentley College (1917) andBrandeis University (1948) inWaltham.Lowell, to the north, is the nation’s first planned industrialcommunity (incorporated town, 1826). Connected by Battle Road,Lexington andConcord were the first battlefields of theU.S. War of Independence. Several notable 19th-century writers lived in the county:Henry Wadsworth Longfellow,Nathaniel Hawthorne,Ralph Waldo Emerson,Henry David Thoreau, andLouisa May Alcott.

Cities such asSomerville,Everett,Woburn,Malden,Marlborough, and Melrose help place Middlesex among the 20 most populous counties in theUnited States. It is also one of the nation’s leading manufacturing counties. The main economic activities are textile manufacture, agriculture, and high-tech businesses, particularly aerospace and defense operations. Area 824 square miles (2,133 square km). Pop. (2000) 1,465,396; (2010) 1,503,085.