  We perform a systematic study of physical properties and distribution of
neutral and ionised gas in the halo of the Milky Way (MW). Beside the large
neutral intermediate- and high-velocity cloud (IVC, HVC) complexes there exists
a population of partly ionised gaseous structures with low-column densities
that have a substantial area filling factor. The origin and nature of these
structures are still under debate. We analyse the physical parameters of the MW
halo gas and the relation to quasar (QSO) metal-absorption line systems at low
and high redshifts. For this purpose we combine new HI 21-cm data from the
EBHIS and GASS surveys with optical quasar absorption line data to study the
filling factor and distribution of these gaseous clouds in the halo at HI
densities below 10^19 1/cm^2. This study is important to understand the
evolution of the MW in particular and the gas accretion mechanisms of galaxies
in general.
