The Barons Baltimore were Catholic, and George Calvert, 1st Baron Baltimore, originally intended that the colony be a haven for English Catholics, though for most of its history Maryland has had a majority of Protestants.

[1] Though formally an independent city rather than a county, the City of Baltimore is considered the equal of a county for most purposes and is functionally a county-equivalent in most respects.

[3] Within Maryland the county is the default unit of local government.

[4] Many of the state's most populous and economically important communities, such as Bethesda, Silver Spring, Columbia, and Towson are unincorporated and receive their municipal services from the county.

[9] The Federal Information Processing Standard (FIPS) code, which is used by the United States government to uniquely identify states and counties, is provided with each entry.