  We present results of an ongoing program to study the gaseous extent of
galaxies and the origin of QSO absorption line systems. For \lya absorption
systems, we find that absorption equivalent width depends strongly on galaxy
impact parameter and galaxy $B$-band luminosity, and that the gaseous extent of
individual galaxies scales with galaxy $B$-band luminosity as $r\propto
L_B^{0.40\pm0.09}$. Applying the results to galaxies in the Hubble Deep Field
to calculate the predicted number density of \lya absorption lines as a
function of redshift and comparing it with observations, we find that at least
50% and perhaps as much as 100% of \lya absorption systems with $W\apg0.32$ \AA
can be explained by the extended gaseous envelops of normal galaxies. The
anti-correlation analysis has also been performed on \civ absorption line
systems, and the results show that the ionized gas cross section scales with
galaxy $B$-band luminosity as $r\propto L_B^{0.76\pm0.26}$.
