  The interstellar medium (ISM) is a very complex medium which contains the
matter needed to form stars and planets. The ISM is in permanent interaction
with radiation, turbulence, magnetic and gravitational fields, and accelerated
particles. Everything that happens in this medium has consequences on the
dynamics and evolution of the Galaxy, resulting the link that relates the
stellar scale with the galactic one. Thus, the study of the ISM is crucial to
advance in the knowledge of stellar and galactic astrophysics. In this article
I present a summary of what we know about the physics and chemistry of this
medium, giving an special emphasis on star formation, and how the processes
related to the stars birth and evolution interrelate with the environment that
surrounds them.
