The American Association of Variable Star Observers (AAVSO) completes 100 years of significant contributions to science in 2011. This centennial history makes available for the first time an accurate and thorough institutional history for scholars while celebrating the achievement for AAVSO members.

The AAVSO is a private, nonprofit international scientific research and educational organization of amateur and professional astronomers who are interested in stars that change in brightness − variable stars. Founded in Connecticut in 1911 to coordinate variable star observations made largely by amateur astronomers, by 1919 the AAVSO had become affiliated with Harvard College Observatory in Cambridge, Massachusetts. In 1954, the AAVSO once again became independent of Harvard and opened its headquarters in Cambridge. Today, with more than 1200 members in 56 countries and a database of more than 19 million observations, the AAVSO is the world's largest association of variable star observers.

Over the course of this century, the scientific agenda of the AAVSO changed, slowly at first, adding a few nonvariable star–related scientific observing projects to its basic program of long-period variable star observation. Over time, a few new categories of variable stars were added, gradually broadening the numbers and types of variable stars observed. In the past four decades, the changes have come more rapidly as more sophisticated instruments became available and especially as the needs of the professional astronomical community broadened. Today, the AAVSO Headquarters and its observers play key roles in the real-time coordination of orbiting observatories managed by NASA and other agencies.