With the advent of the AAO-UKST Galactic Plane
H
survey and
the several other Galactic surveys discussed in this volume, an
opportunity arises for progress in understanding the origin,
structure, and ionization of DIG through the comparison of these
surveys with observations of external edge-on galaxies. These
galaxies offer the advantage of allowing the entire DIG layer to be
quickly imaged (albeit at relatively low linear resolution), while
avoiding the difficulty inherent in Milky Way observations of
translating angular heights of emission to physical heights. Studies
of the diffuse ionized layer of the Milky Way will allow, for example,
the vertical structure of the layer and the dependence of line ratios
on height from the Galactic plane and on the underlying disk
environment to be compared with results from external edge-ons. The
importance of DIG as an ISM component and the leverage it provides on
understanding the ISM as a whole is discussed elsewhere in this volume
by Reynolds, Ferrara, Elmegreen and others. The goal of this review
is to summarize our understanding of DIG in edge-on galaxies at the
time when these surveys are getting underway.