county, south-central Connecticut, U.S. It is bordered to the south by Long Island Sound and to the southwest by the Hammonasset River, and the Connecticut River bisects the county from north to south. Other waterways are the Menunketesuck River, the Moodus Reservoir, and Bashan Lake. The topography is mostly upland terrain, with river valleys and coastal lowlands. Parklands include Cockaponset State Forest, Meshomasic State Forest Preserve, and Hurd and Devil's Hopyard state parks.

The Western Nehantic Indians inhabited the area when John Winthrop the Younger established the Puritan (Puritanism) settlement of Saybrook in the 1630s. Later renamed Old Saybrook, it was where David Bushnell (Bushnell, David) invented and built the Turtle (1775), a submarine used during the American Revolution. The county was formed in 1785 and named for Middlesex, Eng. Essex, a centre of shipbuilding from the 1720s to the 1840s, was raided during the War of 1812 (1812, War of). There is no county seat because the state abolished county government in 1960. The largest city is Middletown, home of Wesleyan University (founded 1831). East Haddam contains the Goodspeed Opera House (built 1876); Gillette Castle (built 1914–19) is located nearby.

Principal industries are agriculture, printing, and the manufacture of aircraft parts. Middlesex has the smallest area of any county in Connecticut. Area 369 square miles (956 square km). Pop. (1990) 143,196; (1996 est.) 148,143.

county, northeastern Massachusetts, U.S., west and northwest of Boston and bordered on the north by New Hampshire. The county consists of an upland region drained by the Merrimack, Nashua, Assabet, Concord, Sudbury, and Shawsheen rivers. Other waterways include Whitehall and Cambridge reservoirs, Lake Cochituate, and historic Walden 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, Eng. The county seat is Cambridge, the home of Harvard University (founded 1636) and the Massachusetts Institute of Technology (1861). Other institutions of higher education include Boston College (1863) in Chestnut Hill–Newton, Tufts University (1852) in Medford, and Bentley College (1917) and Brandeis University (1948) in Waltham. Lowell, to the north, is the nation's first planned industrial community (incorporated town, 1826). Connected by Battle Road, Lexington and Concord were the first battlefields of the U.S. War of Independence (American Revolution). Several notable 19th-century writers lived in the county: Henry Wadsworth Longfellow (Longfellow, Henry Wadsworth), Nathaniel Hawthorne (Hawthorne, Nathaniel), Ralph Waldo Emerson (Emerson, Ralph Waldo), Henry David Thoreau (Thoreau, Henry David), and Louisa May Alcott (Alcott, Louisa May).

Cities such as Somerville, Everett, Woburn, Malden, Marlborough, and Melrose help place Middlesex among the 20 most populous counties in the United 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; (2007 est.) 1,473,416.

county, east-central New Jersey, U.S., bounded by the Millstone River to the southwest, the Raritan River to the northwest, the Rahway River to the northeast, and Raritan Bay to the east. It consists largely of a coastal lowland. Other bodies of water include Carnegie and Farrington lakes and the South River. Forested areas contain oak and hickory. Among the recreational areas are Cheesequake and Edison state parks.

In the 17th century Dutch settlers purchased lands in the region from the Delaware Indians; Scottish, English, and French also were among the early white settlers. The county was the site of several military engagements during the American Revolution. New Brunswick, the terminus of the Delaware and Raritan Canal (completed 1834), is the seat of the county government and of Rutgers, The State University of New Jersey (founded 1766). Perth Amboy, the northern terminus of the Camden and Amboy Railroad (completed 1834), is connected by bridge to Staten Island, N.Y. The city served (1686–1702) as the capital of the East Jersey colony and later as a provincial capital. Inventor Thomas A. Edison (Edison, Thomas Alva) maintained laboratories in Menlo Park (1876–87), where he invented the phonograph (1877), the incandescent lamp (1879), and hundreds of other items.

Middlesex, formed in 1683, is one of New Jersey's original counties. It is named for Middlesex, Eng. Other communities are Woodbridge, Edison, Piscataway, East Brunswick, and Sayreville. The principal components of the economy are manufacturing (especially chemicals), services (business and health), and trade (retail and wholesale). Area 311 square miles (805 square km). Pop. (2000) 750,162; (2007 est.) 788,629.

historic county of southeastern England, incorporating central London north of the River Thames and surrounding areas to the north and west. Most of Middlesex, for administrative purposes, became part of Greater London in 1965.

The River Thames (Thames, River) was the key to the history of Middlesex. From about 8000 BCE traders and settlers used the Thames as their waterway. During the Iron Age (c. 500 BCE) settlements existed at Brentford and Heathrow. In the 1st century BCE Belgic tribes had established themselves in southeastern England, and Middlesex formed part of the Catuvellauni territory. The Romans set up outposts at what became Staines and Brentford. In the early 5th century CE the Saxons began to colonize the area. Positioned as it was between the East and West Saxons, the region soon obtained its modern name (meaning “middle Saxons”); the earliest written record of it is in the form Middelseaxan, in a charter of 704.

From early times Middlesex was dominated by the City of London (London, City of), which in the 12th century obtained the right of appointing the sheriff of the county. It was for centuries the county retreat of royalty and wealthy London merchants. The most outstanding building is the royal palace of Hampton Court; other mansions include Osterley, Syon, and Swakeleys. During early modern times, the urban area of London expanded beyond the boundaries of the City of London into parts of Middlesex such as the present-day London boroughs of Tower Hamlets, Westminster (Westminster, City of), Camden, and Islington. By the late 19th century, London had grown to encompass all of southeastern Middlesex.

In 1888 the new County of London assumed the administration of 50 square miles (130 square km) of Middlesex, with 2.5 million inhabitants, and the new administrative county of Middlesex governed the remaining area. The administrative county acquired its own sheriff, and the site of its traditional courthouse in Parliament Square, Westminster (Westminster, City of), was deemed still in the county of Middlesex for the purpose of holding sessions of the peace.

During the early 20th century, suburban London expanded to cover most of Middlesex. A new metropolitan county, Greater London, established (April 1, 1965) under the 1963 London Government Act incorporated most of the area of Middlesex, along with parts of neighbouring counties. Outer London boroughs created wholly or in part from former Middlesex authorities include Hounslow, Hillingdon, Ealing, Brent, Harrow, Haringey, Enfield, Richmond upon Thames, and Barnet. The inner London boroughs of Camden, Hackney, Hammersmith and Fulham, Islington, Kensington and Chelsea, Tower Hamlets, Westminster, and the City of London also lie within the boundaries of the historic county of Middlesex. While most of Middlesex lies in the metropolitan county of Greater London, the urban districts of Staines and Sunbury-on-Thames lie within the administrative county of Surrey, and Potters Bar in Hertfordshire. Under the Administration of Justice Act (1964) the Middlesex area of London was deemed a county for purposes of law. The name Middlesex continues to be used for postal districts and in the names of many county institutions and organizations.