  The study of open clusters has a classic feel to it since the subject
predates anyone alive today. Despite the age of this topic, I show via an ADS
search that its relevance and importance in astronomy has grown faster in the
last few decades than astronomy in general. This is surely due to both
technical reasons and the interconnection of the field of stellar evolution to
many branches of astronomy. In this review, I outline what we know today about
open clusters and what they have taught us about a range of topics from stellar
evolution to Galactic structure to stellar disk dissipation timescales. I argue
that the most important astrophysics we have learned from open clusters is
stellar evolution and that its most important product has been reasonably
precise stellar ages. I discuss where open cluster research is likely to go in
the next few years, as well as in the era of 20m telescopes, SIM, and GAIA. Age
will continue to be of wide relevance in astronomy, from cosmology to planet
formation timescales, and with distance errors soon no longer a problem,
improved ages will be critically important to many of the most fascinating
astrophysical questions.
