  The Galactic Diffuse X-ray Emission (GDXE) in the Milky Way Galaxy is
spatially and spectrally decomposed into the Galactic Center X-ray Emission
(GCXE), the Galactic Ridge X-ray Emission (GRXE) and the Galactic Bulge X-ray
Emission (GBXE). The X-ray spectra of the GDXE are characterized by the strong
K-shell lines of the highly ionized atoms, the brightest are the K-shell
transition (principal quantum number transition of $n=2\rightarrow1$) of
neutral iron (SXV-He$\alpha$), He-like iron (FeXXV-He$\alpha$) and He-like
sulfur (SXV-He$\alpha$) lines. Accordingly, the GDXE is composed of a
high-temperature plasma of $\sim$7 keV (HTP) and a low-temperature plasma of
$\sim$1 keV (LTP), which emit the FeXXV-He$\alpha$ and SXV-He$\alpha$ lines,
respectively. The FeI-K$\alpha$ line is emitted from nearly neutral irons, and
hence the third component of the GDXE is a Cool Gas (CG). The FeI-K$\alpha$
distribution in the GCXE region is clumpy (FeI-K$\alpha$ clump), associated
with giant molecular cloud (MC) complexes, Sagittarius A, B, C, D and E, in the
central molecular zone. The origin of the FeI-K$\alpha$ clumps is the
fluorescence and Thomson scattering from the MCs irradiated by past big flares
of the super massive black hole Sagittarius A$^*$. The scale heights and
equivalent widths of the FeI-K$\alpha$, FeXXV-He$\alpha$, and FeXXVI-Ly$\alpha$
($n=2\rightarrow1$ transition of H-like iron) lines are different among the
GCXE, GBXE and GRXE. Therefore the structure and origin are separately
examined. This paper overviews the research history and the present
understandings of the GDXE, in particular focus on the origin of the HTP and CG
in the GCXE.
