NOTE: not all past Census reports have been digitized at this time.
Census.gov does offer historical data online, some in PDFs of original publications, and some using other data delivery methods. Remember that many Census tables from recent Censuses include comparison data from earlier years. And there are some papers and other publications on specific topics which may have a historical focus. To find these you can try the web site's Search function, using the words historical AND (be sure to type "and" in all capitals) your topic (e.g., "Foreign-Born," "Housing Units," etc.).
The Blume Library is reducing its collection of printed Census reports, since they are becoming more easily and reliably available online. Records in the Library Catalog contain links to online versions of many printed reports that have been discarded.
A quick way to get population counts (in a plain text file) for counties from 1900 to 1990 is by plugging your state's two-digit postcode abbreviation into this URL, which gives the table for Colorado:
Actual census enumeration forms, which can be useful in genealogical research, are not available at Census.gov, but rather from the National Archives. They become available 70 years after the Census date; thus, at present, records from the 1790-1930 Censuses are available.
A section of GODORT's GITCO Census 2000 Toolkit deals with Historical Census information. It provides links to data, guides and reference materials.
What's in the FTP file repository?
The Census Bureau uses this system to store older Census files (for example, the 1990 information that used to be on American FactFinder will not migrate to the new AFF, but will be kept here), larger files (such as Block Statistics), and American Community Survey tables (presumeably deemed not as popular) not available through the ACS interface. Because of the storage here of older Census material, using the FTP site might be necessary to find historical data.
However, it is possible for folks without this expertise or experience (like me) to find data in the repository. Here are some tips for doing so. (Thanks to Steve Barker, from the Oklahoma State Data Center, for this information.)
Tips for using the FTP file repository
Don't be intimidated by the gopher-reminiscent look to the file list. Just start drilling down. Some abbreviations in the top-level file names:
In drilling down through the file heirarchy on the FTP site, look for README files and others (especially PDF files) containing words such as:
These files can provide valuable information to help find the tables you need and understand the data they represent. Note that different parent directories contain different types of sub-directories and sub-files, so that a helpful file found in one directory might not appear in another one, or might be there, but be named something different.
The Census Bureau has an entire division whose job is make estimates so that researchers and policy-makers can know how the population is changing in between the decennial censuses. Their estimates portal (see the link below) includes links to some historical data back to 1900. They make large datasets available for download, as well as extensive information on methodologies used to arrive at the estimates.
Actual census enumeration forms, which can be useful in genealogical research, are not available at Census.gov, but rather from the National Archives. They become available 70 years after the Census date; thus, at present, records from the 1790-1940 Censuses are available.
Note: forms for all Census years don't contain the same information. For instance, until 1850, the only persons named on the forms were heads of households. Also, information on "enslaved persons" counted on Censuses before and during the Civil War, is sketchy and variable. And much information from the 1890 Census was destroyed in a fire.
More about historical forms:
The National Archives has made the 1940 Census forms available digitally.
The Archives folks have provided a step-by-step guide to finding information in this collection, as well as an informative video that describes the monumental task of digitizing the microfilmed Census records.