  The Warm Ionized Medium (WIM), also referred to as Diffuse Ionized Gas,
contains most of the mass of interstellar medium in ionized form, contributing
as much as 30% of the total atomic gas mass in the solar neighborhood. The
advent of CCDs has enabled unprecedented study of this medium in external
galaxies, probing a variety of environments. In particular, we can derive the
morphology of the WIM, its distribution across disks, and the correlation with
other Population I material. Spectroscopy of the WIM makes it possible to test
various ionization models. I will review here our current understanding of the
properties of the WIM in spiral galaxies. A perhaps unexpected result is that
the H-alpha emission from the WIM contributes about 40% of the total observed
H-alpha luminosity from spirals. This places severe constraints on possible
sources of ionization, since only photo ionization by OB stars meets this
requirement. Spectroscopic measurements of forbidden line strengths appear in
reasonable agreement with photo ionization models. It is not yet clear if the
Lyman continuum photons that ionize the WIM are mostly from OB stars located
inside traditional HII regions, or from field OB stars.
