  Almost 80 years have passed since Trumpler's analysis of the Galactic open
cluster system laid one of the main foundations for understanding the nature
and structure of the Milky Way. Since then, the open cluster system has been
recognised as a key source of information for addressing a wide range of
questions about the structure and evolution of our Galaxy. Over the last
decade, surveys and individual observations from the ground and space have led
to an explosion of astrometric, kinematic and multiwavelength photometric and
spectroscopic open cluster data. In addition, a growing fraction of these data
is often time-resolved. Together with increasing computing power and
developments in classification techniques, the open cluster system reveals an
increasingly clearer and more complete picture of our Galaxy. In this
contribution, I review the observational properties of the Milky Way's open
cluster system. I discuss what they can and cannot teach us now and in the near
future about several topics such as the Galaxy's spiral structure and dynamics,
chemical evolution, large-scale star formation, stellar populations and more.
