  Partially ionized plasmas are found across the Universe in many different
astrophysical environments. They constitute an essential ingredient of the
solar atmosphere, molecular clouds, planetary ionospheres and protoplanetary
disks, among other environments, and display a richness of physical effects
which are not present in fully ionized plasmas. This review provides an
overview of the physics of partially ionized plasmas, including recent advances
in different astrophysical areas in which partial ionization plays a
fundamental role. We outline outstanding observational and theoretical
questions and discuss possible directions for future progress.
