In 2012, 1940 census data was made available. The first U.S. census was actually in 1790. Here's a short guide to what's available for genealogists.
The 1940 census data has just been added. It was witheld till recently to protect the personal information of persons still living (census data is generally private for just over seventy years; the 1950 and later census data is not yet available for genealogical research). So where do you begin?
The first U.S. census was in 1790, but only a portion of Eastern counties were included. Prior to 1850, many of today's states and counties were not yet part of the United States. In addition, only the names of heads of household are listed on the census prior to 1850.
1890: Not There
Most of the 1890 U.S. census was burned in a fire, so 1890 not the best year to look for someone in the census either. There are fortunately some state censuses from the 1880s and 1890s that have survived (the state censuses are, like the U.S. census, taken every ten years, albeit somewhere in the middle of each decade). State censuses that survive from this period include those of Florida, Iowa, Minnesota, Michigan, New York, Rhode Island, and Wisconsin.(For more, see Genealogy Research Guides, "State Census Records.")Starting
U.S. census data is perhaps most helpful for locating ancestors whose lifespans included periods between 1850 and 1940. However, it's often in fact easier to start with a more recent census (1900 through 1940) and then trace households backwards.
Beginning in 1900, the census listed each occupant in the household, his or her age, his or her occupation (occupations could of course include "keeping house," for women, or "at home," for children below school age), and his or her relationship to the head of household. In addition, the census listed each person's birth place (often just the state is listed but sometimes the census taker provided more details; for persons born outside of the United States, just the country is listed), along with the birth places of each parent, and finally, if the person immigrated, his or her immigration date.
A birth place is helpful in helping you to know where to look for an ancestor on an earlier census. However, if the person you are trying to trace back is a married woman, the name changes over time (even in some cases, such as adoption, the last names of men change over time!), and that can be a bit of (but not an insurmountable) barrier. Marriage records may allow you to trace a female ancestor. Name change records, often recorded by the county clerk's office, can help when an ancestor has changed a surname following immigration
Parents' birth information helps as well. Say, for example, that you locate an ancestor on the 1930 Chicago, Cook County, Illinois census who was aged forty at the time. The census data states that he was born in Ohio. The census also lists his mother as born in Ireland and his dad as born in Ohio. If you find a child of the same name albeit age ten on a 1900 Ohio census, with a mother born in Ireland and a dad born in Ohio, you've probably found your ancestor.
The 1880 census does not have data about arrival dates of immigrants but has otherwise much the same data as the later censuses. The 1880 census images, with indexing, are provided free of charge by ancestry.com. (With indexing however, there can be transcription errors; for one ancestor of mine, a "q" in the second name was transcribed as a "p;" when I failed to locate my ancestor; I tried searching under first names, ages, and counties, and found my ancestor immediately.)
The 1850, 1860, and 1870 censues do not indicate parents' birth places but do indicate the birth place of each person indexed, as well as the name. Persons who were slaves in 1850 or 1860 are listed on the slave schedules, and only first names are listed. Native Americans were not in many cases censused until 1860. (As noted above, prior to 1850, only names of heads of households were listed; other persons in each householed were counted by age and sex, but no names were provided. Note that today's censuses, with the "short forms," are once again providing fewer genealogical details. These contemporary censuses won't in any case be available to genealogists until seventy years have elapsed since the census date.)
Sleuthing Household Relationships
A person's relationship to the head of household is also not provided on the censuses before 1880, but you may nevertheless find clues. Even persons of different names living in a household (unless the latter are itinerant workers or borders) may be somehow related. I've surmised that the mother of one ancestor's maiden name was "Smith" (at last, not a very helpful last name in terms of searching in a large metropolitan area like she lived in) by the fact that several young Smiths are listed on the 1850 census as living in her and her husband's household. They could have been borders, but the middle initial for her son was "S" (I still have to find out whether or not his middle name was "Smith").
Also, because her son's surname, "Angell," did not match her and her husband's surname, I assume she was married previously and that her previous married name was Angell (interestingly the son is ultimately listed on a later census with a child who shares a first and middle name with this same woman, so I have further confirmation that she was the natural mother; he did ultimately change his last name from Angell to the woman's new married name, but he never gave a child to my knowledge the first name of the husband).
It actually took some sleuthing to find this household in 1850, because initially, with no clues about the parents except the state they were born in (Rhode Island, listed on the 1880 census of the household of the by-then adult son), I was looking for a family whose son had the same last name as the parents. Fortunately, a family of the same last name, also from Rhode Island, who was listed on the 1870 census as living in the same town as the son was listed as living in in 1880, provided one of the clues I needed. This family was not the family, but when I traced it back to Rhode Island, I found the real family right next door (the two neighbors with the same last name may have been brothers), with a son with the right first name and middle initial, and of the right age. (Next-door neighbors will appear together on the census. This kind of sleuthing works well when you have the actual images, although some transcriptions may also list adjacent lots on the same page, with neighbors listed in adjacent rows.) I confirmed that I had gotten this right when I found a list of court records that included the name change petition.
Tracing Former Slaves
According to many sources, after the Civil War, many slaves for at least a decade remained near to the plantations where they had worked as slaves. Thus, if you find your ancestors on the 1880 census or better yet, the 1870 (but the 1870 census of course will not indicate household relationships), it's likely that the plantation they worked on in 1860 was somewhere nearby, and thus one option for tracing backwards is to try to locate people of the same first name, in the right age group, on a nearby plantation on the 1860 census.
One possibility of course is to locate ancestors on a plantation where the owner had the same last name that's listed on the later census for the ancestors you are trying to trace. However, slaves did not always get last names from the plantations where they worked. According to John Baker, Jr. ("Surnames Used by African American Slaves," The Washingtons of Wessington Plantation blog), ex-slaves might also take as surnames the names of other former owners, or second names the slaves had chosen to distinguish one another on large plantations, as well as possibly surnames of actual fathers.
Where to Get U.S. Census Data Online
Besides the free 1880 census provided by ancestry.com, ancestry provides all U.S. censuses and some state censuses to subscribers. Some libraries subscribe to ancestry. In addition, Rootsweb and Family search provide transcriptions (not images) of selected census data free of charge.