Maryland ranked 18th in population in the US with an estimated total of 5,458,137 in 2002, an increase of 3.1% since 2000. Between 1990 and 2000, Maryland's population grew from 4,781,468 to 5,296,486, an increase of 10.8%. The population is projected to reach 5,467,000 by 2005 and 6.3 million by 2025.

In 2000 the median age was 36. Persons under 18 years old accounted for 25.6% of the population while 11.3% were age 65 or older.

The state's population doubled between 1940 and 1970 and increased 7.5% between 1970 and 1980. The enormous expansion of the federal government and exodus of people from Washington, D.C., to the surrounding suburbs contributed to the rapid growth that made Maryland the 17th most populous state in 1980, with 4,216,446 residents. There was an increase of 13.4% between 1980 and 1990, when Maryland held the 19th ranking, with 4,781,468 people. The population density in 2000 was 541.9 persons per sq mi, the 5th-highest among the 50 states.

Almost all the growth since World War II has occurred in the four suburban counties around Washington, D.C., and Baltimore. Metropolitan Baltimore, embracing Carroll, Howard, Hartford, Anne Arundel, and Baltimore counties, expanded from 2,244,700 to 2,491,254 between 1984 and 2000; the city of Baltimore, on the other hand, declined from 763,570 to 736,000 during the same period, and to an estimated 638,614 in 2002. Baltimore is the state's only major city; several west-central counties belong to the Washington metropolitan area, and Cecil County, in the northeast, is part of metropolitan Wilmington, Delaware.