  Thick layers of warm, low density ionized hydrogen (i.e., the warm ionized
medium or WIM) in spiral galaxies provide direct evidence for an interaction
between the disk and halo. The wide-spread ionization implies that a
significant fraction of the Lyman continuum photons from O stars, produced
primarily in isolated star forming regions near the midplane and often
surrounded by opaque clouds of neutral hydrogen, is somehow able to propagate
large distances through the disk and into the halo. Moreover, even though O
stars are the source of the ionization, the temperature and ionization state of
the WIM differ significantly from what is observed in the classical O star H II
regions. Therefore, the existence of the WIM and observations of its properties
provide information about the structure of the interstellar medium and the
transport of energy away from the midplane as well as place significant
constraints on models.
