  The development of our understanding of the gaseous component of the
interstellar medium is reviewed. During the 20th century our picture of diffuse
material in space has grown from a simple model of isolated clouds in thermal
equilibrium with stellar radiation fields to one of a richly varied composite
of materials with a wide range of physical properties and morphologies. The
solar system interacts with this dynamical interstellar medium, which dominates
the properties of the interplanetary environment. Optical, radio, and UV
astronomy allow us to study the clouds which form the galactic environment of
the Sun. The composition and distribution of interstellar clouds in the disk
and halo tell us about the history of elemental formation in our galaxy, and
the past and future environment of the solar system.
