town (township), Middlesex county, northeastern Massachusetts, U.S. Located just southeast of Lowell and 21 miles (34 km) north of Boston, the town occupies a marshy lowland between the Concord and Merrimack (Merrimack River) rivers. Farmers from neighbouring Billerica settled there during the early 18th century, and a sawmill was established. Commercial flower growing and leather tanning were major economic activities in the 19th and early 20th centuries. A large state-sponsored almshouse was opened in 1854.

Manufacturing (computer systems and electronic equipment) now accounts for the largest share of employment; services and the insurance business are also important. The defense contractor Raytheon (Raytheon Company) maintains a large manufacturing and engineering laboratory in the town. Inc. 1734. Area 21 square miles (54 square km). Pop. (1990) 27,266; (2000) 28,851.