  Open clusters and associations are groups of young stars, respectively bound
and unbound, that share the same origin and disperse over time into the
galactic field. As such, their formation and evolution are the key to
understand the origin and properties of galactic stellar populations. Moreover,
since their members have about the same age, they are ideal laboratories to
study the properties of young stars and constrain stellar evolution theories.
  In this contribution, I present our current knowledge on open clusters and
associations. I focus on the methods used to derive the statistical properties
(IMF, spatial distribution, IMF) of young stars and briefly discuss how they
depend on the environment. I then describe how open clusters can be used as
probes to investigate the structure, dynamics and chemical composition of the
Milky Way. I conclude by presenting the Gaia mission and discuss how it will
revolutionize this field of research.
