Coordinates: 39°24′N 76°36′W / 39.4°N 76.6°W / 39.4; -76.6

Baltimore County is a county located in the northern part of the US state of Maryland. In 2010, its population was 805,029. It is part of the Baltimore-Washington Metropolitan Area. Its county seat is Towson. The name of the county was derived from the barony of the Proprietor of the Maryland colony, Cæcilius Calvert, 2nd Baron Baltimore, in County Longford, Ireland. Baltimore County does not include the city of Baltimore, which left the county to become an independent city in 1851.

Much of Baltimore County is suburban in character, straddling the border between the Piedmont plateau and, in the southern regions of the county, the Atlantic coastal plain. Northern Baltimore County is primarily rural, with a landscape of rolling hills and deciduous forests characteristic of the Southeastern mixed forests.

Among the county's major employers are the Social Security Administration, which has its national headquarters in Woodlawn, and Black & Decker in Towson. During World War II, the Glenn L. Martin Company in Middle River had 53,000 employees manufacturing airplanes for the war effort and Bethlehem Steel had more than 30,000 workers at its sprawling Sparrows Point steel mill. Of the 410,100 persons in the county's workforce as of 2009, 25% are employed in the fields of education, health, and human services, and 10% in retailing, with less than 1% in agriculture.

Baltimore ( /ˈbɒltɨmɔr/, colloquially /ˈbɔl.mɔr/) is the largest city in the U.S. state of Maryland. It is located in the central area of the state along the tidal portion of the Patapsco River, an arm of the Chesapeake Bay. The independent city is often referred to as Baltimore City to distinguish it from surrounding Baltimore County.

Founded in 1729, Baltimore is the largest seaport in the Mid-Atlantic United States and is situated closer to Midwestern markets than any other major seaport on the East Coast. Baltimore's Inner Harbor was once the second leading port of entry for immigrants to the United States and a major manufacturing center. After a decline in manufacturing, Baltimore shifted to a service-oriented economy.

At 620,961 residents in 2010, Baltimore's population has decreased by one-third since its peak in 1950. The Baltimore Metropolitan Area has grown steadily to approximately 2.7 million residents in 2010; the 20th largest in the country. Baltimore is also a principal city in the larger Baltimore–Washington combined statistical area of approximately 8.4 million residents.

Maryland (ⁱ/ˈmɛrɨlənd/) is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east. Maryland was the seventh state to ratify the United States Constitution, and three nicknames for it (the Old Line State, the Free State, and the Chesapeake Bay State) are occasionally used. Maryland is the 9th smallest state by area, but the 19th most populous and the 5th most densely populated of the 50 United States. The state's most populated city is Baltimore. Its capital is Annapolis. It was named after Queen Henrietta Maria.

Maryland has an area of 12,406.68 square miles (32,133.2 km²) and is comparable in overall area with the European country of Belgium (11,787 square miles (30,530 km²)). It is the 42nd largest/9th smallest state, and is closest in size to Hawaii (10,930.98 square miles (28,311.1 km²)), the next smallest state. The next largest state, Maryland's neighbor West Virginia, is almost twice the size of Maryland (24,229.76 square miles (62,754.8 km²)).