The Census Bureau does not provide data that's more than a few decades old on its website; use the sources below. Questions on surveys, subject categories, and census geographies change over time. Statistical census geography is revised every ten years, while legal geography is updated on an on-going basis. Nominal data is reported the same way that it was originally published. Normalized data has been modified so that data from the past has been transformed to fit present day boundaries.
The modern decennial census included at least two forms up to and including the year 2000: a short form collected basic data from 100% of the population, and a long form captured more detailed characteristics from a large sample. Beginning in 2010 the decennial census consisted of just a short form, while the long form was supplanted by the American Community Survey that was launched in 2005. Bear this in mind when making recent historical comparisons.
All of the IPUMS sites listed on this page are part of the IPUMS Project at the Population Center at the University of Minnesota. You must create one account to access these resources, but it's free for academic, educational, or non-profit research purposes.
While public summary data is aggregated into subject categories and geography, public microdata consists of anonymized sample records that represent individual responses to questionnaires. Special weighting variables are used to generate estimates from the samples, including cross-tabulations that are not created for the public summary datasets. To protect confidentiality, estimates can only be generated for large geographies like states, the largest metro areas, and Public Use Microdata Areas (PUMAs) that consist of 100k people. Sometimes it's possible to generate data for the largest counties.
Visit the Census Bureau's website and do a search through the various programs (decennial census, ACS, CPS) to find information about microdata for each program and to download raw data files for entire years and series.. The sources listed below are alternatives that allow you to create specific extracts.