The 1940 Census is available online. It is still in the process of being indexed but you can browse specific geographic areas. This census asked a number of questions that were not included in previous censuses. Besides name, age, relationship, and occupation, the 1940 census included questions about internal migration; employment status; participation in the New Deal Civilian Conservation Corps (CCC), Works Progress Administration (WPA), and National Youth Administration (NYA) programs; and years of education.
The 1790-1930 census records containing information on individual households are available on microfilm from a number of sources. Most of the older censuses were handwritten and it can be exceedingly difficult for indexes to list some names correctly.
The U.S. Census is arranged by state - so there is a section of each U.S. Census for Iowa. The ISU Library has microfilm copies of the original census records for 1840-1930 - but only for Iowa counties.
Population Schedules of the...Census of the United States...Iowa
HA 371.5 (Year) C45x - microfilm - Media Center
The Media Center has 1840-1880, 1880 Soundex, 1900, 1900 Soundex, 1910-1930
1890 is not available, the records were destroyed by fire at the National Archives.
This set contains individual handwritten household records as collected by census takers. The information collected varies with each census, but can include name, age, race, sex, marital status, household relationships, physical and mental disabilities, education, occupation, where migrated from, and parents place of birth. The Soundex is a somewhat complicated name index created specifically for use with census records. It allows researchers to search for similar sounding names - to help assist with errors due to illegible handwriting and the general lack of knowledge in how some names should be spelled.
Iowa State Census - the state did publish an independent census outside of the Federal census in some years, e.g., 1856, 1859, 1915, 1925. The 1925 census in particular asked a number of excellent questions that were never asked in other census years. These are, unfortunately, not available at ISU. The Ames Public Library has 1856, 1885 and 1895. They also have the Iowa State Census of 1915 and 1925 for Story County ONLY.
Census of Iowa - was issued from 1856-1925. It does not list individuals living in Iowa, rather it summarizes information about Iowans as reported in the census - e.g., number of widowed people living in x county, the number of farmers, etc.
Other books about the U.S. census, and Iowa census indexes for 1840-1860, are available in the Media Center Reference area (Lower Level).