U.S. Census Bureau Catalog and Guide
Especially useful for historical census volumes
Selected Census Products, below: American Factfinder, Social Explorer, Geolytics, County and City Databooks and print sources.
For additional sources of statistical information, see: Statistics Sources for the Social Sciences
The population is "counted" every 10 years via the decennial census. For population counts (total number of persons) between decennial censuses, see: Population Estimates
The decennial censuses (conducted every ten years) and the American Community Survey (ACS), a rolling survey designed to replace the decennial census Long Form, offer a wide variety of tabulations based on the questions asked. See the tabs above for more information about the data compiled in the decennial censuses and, more recently, in the American Community Survey.