I need to identify all years (since 1900) in which previous U.S.
economic recessions have occured.

Hi cheshamjim

Recession troughs were reached during the following years since 1900. 
The month listed is when the recession "bottomed out."

Dec. 1900 - Aug. 1904 - June 1908 - Jan. 1912 - Dec. 1914 - Mar. 1919
- July 1921 - July 1924  - Nov. 1927 - (From Aug. 1929 through Mar.
1933 the classic Great Depression) - June 1938 - Oct. 1945 - Oct. 1949
- May 1954 - Apr. 1958 - Feb. 1961 - Nov. 1970 - Mar. 1975 - July 1980
- Nov. 1982 - Mar. 1991

Since 1991, there has been an economic recovery followed by another
recession the "trough" of which is yet to be determined, making a
total of 22 "recessions."

The above listing is based on the simple definition - "Periods of
economic expansion are typically called booms; periods of economic
decline are called recessions or depressions." - Quote and figures are
from the website - "Business Cycles, by Christina D. Romer: The
Concise Encyclopedia of Economics: Library of Economics and Liberty" -
( http://www.econlib.org/library/Enc/BusinessCycles.html ) - You will
find a lot more information about economic cycles and their causes
when you visit the site.

If I can add clarification to any of the above, please don't hesitate
to ask.

Search - Google
Key terms - economic cycles usa, recession history usa

Cheers
digsalot