  We have analyzed deep narrow-band H$\alpha$ images and high-resolution
long-slit spectra for a sample of the nearest and brightest late-type galaxies
to study the morphology, physical state, and kinematics of the `Diffuse Ionized
Medium' (`DIM'). We find that the DIM covers most of the star-forming disk, and
is morphologically related to the presence of the giant HII regions. In
addition, the DIM and the giant HII regions differ systematically in their
physical and dynamical state. The DIM is characterized by enhanced emission in
the low-ionization forbidden lines ([OI], [NII], and [SII]), and even the
high-ionization [OIII]$\lambda$5007 line is moderately strong in the DIM. We
verify the inference made by Lehnert & Heckman that the DIM contributes
significantly to the global emission-line ratios measured in late-type
galaxies. We also find that the DIM is more disturbed kinematically than the
gas in the giant HII regions. The intrinsic FWHMs of the H$\alpha$ and
[NII]$\lambda$6584 lines range from 30 to 100 km s$^{-1}$ in the DIM compared
to 20-50 km s$^{-1}$ in HII regions. The high-ionization gas in the DIM is even
more kinematically disturbed than the low-ionization gas: the
[OIII]$\lambda$5007 lines have intrinsic FWHMs of 70-150 km s$^{-1}$. The
differing kinematics implies that `the DIM' is not a single monolithic phase of
the ISM. Instead, it may consist of a `quiescent DIM' with a low
ionization-state and small scale-height (few hundred pc) and a `disturbed DIM'
with a high ionization state and moderate scale-height (0.5 to 1 kpc). We argue
that the quiescent DIM is most likely photoionized by radiation leaking out of
giant HII regions, while the disturbed DIM is most likely heated by the
mechanical energy supplied by supernovae and stellar winds.
