  The Ara OB1a association is one of the closest sites where triggered star
formation is visible for multiple generations of massive stars. At about 1.3
kpc distance, it contains complex environments including cleared young
clusters, embedded infrared clusters, CO clouds with no evidence of star
formation, and clouds with evidence of ongoing star formation. In this review
we discuss the research on this region spanning the last half-century. It has
been proposed that the current configuration is the result of an expanding wave
of neutral gas set in motion between 10--40 million years ago in combination
with photoionization from the current epoch.
