The county has always been used as the basic Federal census unit. Genealogical research in the census, therefore, begins with identifying the correct county jurisdictions. This work shows all U.S. county boundaries from 1790 to 1920. On each of the nearly 400 maps the old county lines are superimposed over the modern ones to highlight the boundary changes at ten-year intervals. Also included are (1) a history of census growth; (2) the technical facts about each census; (3) a discussion of census accuracy; (4) an essay on available sources for each state's old county lines; and (5) a statement with each map indicating which county census lines exist and which are lost. Then there is an index listing all present-day counties, plus nearly all defunct counties or counties later re-named.