A U.S. population census has been conducted every 10 years since 1790.
All records, up to 1940, are publicly available, however much of the 1890 U.S. Census was destroyed in a 1921 fire in the Commerce Building.
The 1950 U.S. Census, and subsequent decades, are kept confidential to protect the privacy of individuals listed who are likely to still be living. Every 72 years (after enumeration), the Census Bureau releases another Census. The 1940 Census was released April 2, 2012.
Infographic provided by the U.S. Census Bureau.