Census records are confidential for 72 years, but after that anyone with an interest can look at them.
That means that the censuses of 1790 to 1930 are now available, with the 1940 one to be released in 2012.
For a full report on its services, check its website at gov.
Probably the richest sources maintained by the National Archives are the records of the censuses, which the United States has conducted every ten years, without missing a single one, since 1790.
States and cities also maintain archives and these, like the National Archives, usually allow visitors to look at their documents.
They may also provide copies by mail, for a fee, to anyone who can be specific about the information sought.
To use them, though, you need to have a pretty good idea of when the event occurred.