The diffuse ionized gas (DIG), also referred to as the Reynolds layer,
is a nearly fully ionized gas with a mean midplane density of
and a scale height of 
. This layer has an
average filling factor of 
with the possibility that the
filling factor increases from 
in the galactic plane to
at 
(Kulkarni & Heiles 1987; Reynolds 1991,
1993). The average mean density of a clump in this layer is
while the mean density of a clump in the Galactic plane
is 
.
Table 1 summarizes the observed properties in the DIG. The majority
of these measurements have been made near the Galactic midplane within
of the Sun. Observed limits of 
emission from
the DIG indicate that the hydrogen is at least 
ionized, and
that 
ionization is probable (Reynolds 1989).
The energy
required to keep the DIG ionized is 
, corresponding
to 
hydrogen ionizing photons per 
per sec
in the Galactic disk ([Reynolds 1992]). This corresponds to 
of
the Lyman continuum photons of Galactic OB stars or of the
kinetic energy injected by Galactic supernovae into the interstellar
medium (ISM) ([Reynolds 1993]).
The electron
temperature of the DIG has been constrained to be in the range 
([Reynolds 1985a]), with an average of
. Reynolds & Tufte (1995)
found an upper limit of 
which indicates that
the ionizing spectrum incident on the DIG is relatively soft and
corresponds to a helium-to-hydrogen ionization fraction of 
. This is consistent with radio
recombination line measurements of H and He toward the inner Galaxy
which produce 
(Heiles et al.\
1996). The observed ranges of 
and
require a much harder ionizing
spectrum if photoionization is to explain these line ratios
([Reynolds 1985a]).
It appears that the physical conditions in the DIG cannot be explained by simple photoionization models and that other physical processes may be important in the DIG. It has been determined that the DIG in NGC 891 also cannot be explained by simple photoionization models ([Rand 1997]). Previous models developed by Mathis (1986) and Domgörgen & Mathis (1994) have attempted to explain the conditions in the DIG but have not been entirely successful (Reynolds & Tufte 1995; [Rand 1997]). Raymond (1976) and Shull & McKee (1979) have developed models in which shocks in the DIG predict the observed line ratios. The shock models, however, produce results that are inconsistent with the observed linewidths ([Reynolds 1985a], 1985b).