  The interstellar medium (ISM) can be thought of as the galactic atmosphere
which fills the space between stars. When clouds within the ISM collapse, stars
are born. When the stars die, they return their matter to the surrounding gas.
Therefore the ISM plays a vital role in galactic evolution. The medium includes
starlight, gas, dust, planets, comets, asteroids, fast moving charged particles
(cosmic rays) and magnetic fields. The gas can be further divided into hot,
warm and cold components, each of which appear to exist over a range of
densities, and therefore pressures. Remarkably, the diverse gas components,
cosmic rays, magnetic fields and starlight all have very roughly the same
energy density of about 1 eV cm$^{-3}$. All the major constituents (or phases)
of the interstellar medium appear to be identified now, although complete
multi-phase studies are extremely difficult beyond a few thousand parsecs from
the Sun. The interstellar medium is a highly complex environment which does not
lend itself to simple analysis. However, this has not stopped astrophysicists
from producing basic models of the ISM in order to make sense of the great
wealth of data coming in from ground-based telescopes and from satellites.
