A census tract, census area, or census district is a geographic region defined for the purpose of taking a census. Census tracts are usually the size of neighborhoods and encompass anywhere from 2,500 to 8,000 people. Usually these coincide with the limits of cities, towns or other administrative areas; however, the primary concern is making sure that these tracts are made where human populations are prevalent.  In unincorporated areas of the United States these are often arbitrary, except for coinciding with political lines.
The first time small geographic areas were broken up and delineated was for the 1890 census, as a part of a study of vital statistics. They were delineated by population, topography, and housing characteristics and called sanitary districts by the Census Office, which later became the Census Bureau.
In 1905, Dr. Walter Laidlaw originated the concept of permanent, small geographic areas as a framework for studying change from one decennial census to another in neighborhoods within New York City. For the 1910 Census, eight cities — New York, Baltimore, Boston, Chicago, Cleveland, Philadelphia, Pittsburgh, and St. Louis — delineated census tracts (then termed ‘‘districts’’) for the first time. No additional jurisdictions delineated census tracts until just prior to the 1930 Census, when an additional ten cities chose to do so. The increased interest in census tracts for the 1930 Census is attributed to the promotional efforts of Howard Whipple Green, who was a statistician in Cleveland, Ohio, and later the chairman of the American Statistical Association’s Committee on Census Enumeration Areas. For more than twenty-five years, Mr. Green strongly encouraged local citizens, via committees, to establish census tracts and other census statistical geographic areas. The committees created by local citizens were known as Census Tract Committees, later called Census Statistical Areas Committees.
After 1930, the Census Bureau saw the need to standardize the delineation, review, and updating of census tracts and published the first set of census tract criteria in 1934. The goal of the criteria has remained unchanged; that is, to assure comparability and data reliability through the standardization of the population thresholds for census tracts, as well as requiring that their boundaries follow specific types of geographic features that do not change frequently. The Census Bureau began publishing census tract data as part of its standard tabulations beginning with the 1940 Census. Prior to that time, census tract data were published as special tabulations.
For the 1940 Census, the Census Bureau began publishing census block data for all cities with 50,000 or more people. Census block numbers were assigned, where possible, by census tract, but for those cities that had not yet delineated census tracts, ‘‘block areas’’ (called ‘‘block numbering areas’’ [BNAs] in later censuses) were created to assign census block numbers. Starting with the 1960 Census, the Census Bureau assumed a greater role in promoting and coordinating the delineation, review, and update of census tracts. For the 1980 Census, criteria for BNAs were changed to make them more comparable in size and shape to census tracts. For the 1990 Census, all counties contained either census tracts or BNAs.
Census 2000 was the first decade in which census tracts were defined in all counties. In addition, the Census Bureau increased the number of geographic areas whose boundaries could be used as census tract boundaries. It also allowed tribal governments of federally recognized American Indian tribes with a reservation and/or off-reservation trust lands to delineate tracts without regard to State and/or county boundaries, provided the tribe had a 1990 Census population of at least 1,000.
Census tracts are designed to be generally consistent through time. However, when the distribution of people does change, it is important for that to be documented. The Participant Statistical Areas Program (PSAP) was created to encourage local help in updating statistical information. Census tracts are either split, merged, added, or eliminated depending on how the spatial distribution of population changed. This program runs about once every ten years. The data collected is kept and compared to earlier reports. 
- "Census Tracts and Block Numbering Areas". U. S. Census Bureau. 2000-04-19. http://www.census.gov/geo/www/cen_tract.html. Retrieved 2007-12-05.
- "Finding Census Tract Data". Michigan State University. http://libguides.lib.msu.edu/tracts. Retrieved 2017-11-05.
- "Census Tract Program for the 2010 Decennial Census—Final Criteria". Federal Register. U. S. Census Bureau. 2008-05-14. http://www.census.gov/geo/www/psap2010/tract_criteria.pdf. Retrieved 2008-10-14.
- . United States Census Bureau.
- Chapter 10: Census Tracts and Block Numbering Areas, U.S. Census Bureau, Geographic Areas Reference Manual (PDF)
- gCensus: Browse Tract Numbers, Block Group Numbers, and Block Numbers