Map - Baltimore County, Maryland (Baltimore County)

Baltimore County (, locally: or ) is the third-most populous county in the U.S. state of Maryland and is part of the Baltimore metropolitan area. Baltimore County (which partially surrounds, though does not include, the independent City of Baltimore) is part of the Northeast megalopolis, which stretches from Northern Virginia northward to Boston. Baltimore County hosts a diversified economy, with particular emphasis on education, government, and health care. As of the 2020 census, the population was 854,535.

The county is home to multiple universities, including Goucher College, Stevenson University, Towson University, and University of Maryland, Baltimore County.

The name "Baltimore" derives from Cecil Calvert, 2nd Baron Baltimore (1605–1675), the proprietor of the new colony in the Province of Maryland, and the town of Baltimore in County Cork, Ireland. The earliest known documentary record of the county is dated January 12, 1659, when a writ was issued on behalf of the General Assembly of Maryland to its sheriff. The official founding of the county came in 1659, among the now 23 counties of the State of Maryland. This assumes that a certain amount of organization and appointments in the mid-17th century had already occurred. Previously, (old) Baltimore County was known more as a geographical entity than a political one, with its territorial limits including most of northeastern Maryland, which was then the northwestern frontier of the Province and included the present-day jurisdictions of Baltimore, Cecil and Harford Counties, and parts of Carroll, Anne Arundel, Frederick, Howard, and Kent Counties.

In 1674, a proclamation of the Proprietor established the then-extensive boundary lines for old Baltimore County. Over the next century, various segments of the old county were sliced off as population and settlements increased in fringe regions. A portion of northeastern Baltimore County, as well as a portion of northwestern Kent County, was split off to create Cecil County. In 1748, a portion of western Baltimore County, as well as a portion of Prince George's County to the south, were split off to create Frederick County. In 1773, Harford County to the east was split off, and in 1837 another part of western Baltimore County was combined with a part of eastern Frederick County to create Carroll County. After the adjustment of Baltimore County's southern boundary with Anne Arundel County, stated to be the upper Middle and Western Branches of the Patapsco River in 1727, a portion of the county's northwestern area was designated in 1838 as the "Western District" or "Howard District" of Arundel and in 1851 was officially separated to form Howard County.

Before 1674, Baltimore County court sessions were held in private residences, according to sketchy documentary evidence. In 1674, the General Assembly passed "An Act for erecting a Court-house and Prison in each County within this Province". The site of the courthouse, jail and county seat for Baltimore County was evidently "Old Baltimore" near the Bush River on land that in 1773 became part of Harford County.

The exact location of Old Baltimore was lost. It was certain that the location was somewhere on the site of the present-day Aberdeen Proving Grounds (APG), a U.S. Army weapons testing facility. APG's Cultural Resource Management Program attempted to find Old Baltimore, contracting with R. Christopher Goodwin & Associates (Goodwin). Goodwin first performed historical and archival work and coordinated with existing landscape features to locate the site of Old Baltimore. APG's Explosive Ordnance Disposal of Army personnel defused any unexploded ordnance. In 1997–1998. Goodwin dug 420 test pits, uncovering artifacts including a King Charles II farthing coin, and French and English gun flints. An unearthed brick foundation proved to be the remains of the tavern owned by colonist James Phillips. Another prominent landholder in Old Baltimore was William Osbourne, who operated the ferry across the Bush River.

In his article "Migrations of Baltimore Town", Reverend George Armistead Leakin related a letter he had received from Dr. George I. Hays. In that letter, Dr. Hays related an account of a raid by the Susquehannocks who took William Osbourne's oldest son. Osbourne was unsuccessful in an attempt to rescue the boy. The boy was never seen by Osbourne again.

In 1683, the Maryland General Assembly passed "An Act for Advancement of Trade" to "establish towns, ports, and places of trade, within the province." One of the towns established by the act was "on Bush River, on Town Land, near the Court-House". The courthouse on the Bush River referenced in the 1683 Act was in all likelihood the one created by the 1674 Act. "Old Baltimore" was in existence as early as 1674, but no documents describe what may have preceded it.

By 1695, the "Old Baltimore" courthouse had evidently been abandoned. County justices put the site up for sale. Apparently a new courthouse at "Simm's Choice" on the Baltimore County side of Little Gunpowder Falls had been under construction since 1692. In 1700, builder Michael Judd sold it to the county justices. This change of location, away from the Bush River area, reflects the growing economic and political importance of the Gunpowder region. During the next decade, the county seat moved to Joppa.

By 1724, the legislative assembly authorized Thomas Tolley, Capt. John Taylor, Daniel Scott, Lancelot Todd, and John Stokes to purchase 20 acres from "Taylor's Choice," a tract named after John Taylor. The assembly's ordinance directed that the land be divided into 40 lots with streets and alleys to accompany the courthouse and jail erected previously. By 1750, about 50 houses (including a few large two-story brick structures), a church (St. John's Anglican Parish), a courthouse, three stone warehouses, inns, taverns, stores, a public wharf and a "gallows-tree" with an "Amen Corner" with pillories and whipping posts (now located northeast of the City of Baltimore near present-day suburban "Joppatowne" off Harford Road) existed.