Ancestry refers to a person’s ethnic origin or descent, "roots," or heritage, or the place of birth of the person or the person’s parents or ancestors before their arrival in the United States. Some ethnic identities, such as "German" or "Jamaican," can be traced to geographic areas outside the United States, while other ethnicities such as "Pennsylvania Dutch" or "Cajun" evolved in the United States.

The intent of the ancestry question is not to measure the degree of attachment the respondent had to a particular ethnicity. For example, a response of "Irish" might reflect total involvement in an "Irish" community or only a memory of ancestors several generations removed from the individual. A person’s ancestry is not necessarily the same as his or her place of birth; i.e., not all people of German ancestry were born in Germany (in fact, most were not).

Currently, when someone reports more than two groups for their ancestry in the American Community Survey, only the first two ancestries are tabulated.

Some people identify their ancestry as American. This could be because their ancestors have been in United States for so long or they have such mixed backgrounds that they do not identify with any particular group. Some foreign born or children of the foreign born may report American to show that they are part of American society. There are many reasons people may report their ancestors as American, and the growth in this response has been substantial.

The ancestry question was added to the census form in 1980, so the earliest information available from this question is from 1980. Several publications listed in the “Publications” section of this ancestry website contain 1980 and 1990 data.