  We report results from our $z\simeq 5.5$--7 galaxy search in the HST Ultra
Deep Field (UDF). Using the 400-orbit of ACS data, we found 108 plausible
$5.5\leq z\leq 6.5$ (or $z\simeq 6$ for short) candidates to
$m_{AB}(z_{850})=30.0$ mag. The contamination to the sample, either due to
image artifacts or known types of astronomical objects, is likely negligible.
The inferred surface densities of $z\simeq 6$ galaxies are consistent with our
earlier predictions from $m_{AB}(z_{850})=26.5$ to 28.5 mag. After correcting
for detection incompleteness, the counts of $z\simeq 6$ candidates to
$m_{AB}(z_{850})=29.2$ mag suggests that the faint-end slope of the galaxy
luminosity function (LF) at this redshift is likely between $\alpha=-1.8$ and
-1.9, which is sufficient to account for the entire Lyman photon budget
necessary to complete the reionization of the universe at $z\simeq 6$. We also
searched for $z\simeq 6.5$--7 candidates using the UDF NICMOS data, and have
found four candidates to $J_{110}=27.2$ mag. However, the infrared colors of
three candidates cannot be easily explained by galaxies in this redshift range.
We tentatively derive an upper limit to the cumulative surface density of
galaxies at $z\simeq 7$ of 0.36 per arcmin$^2$ to $J_{110}=26.6$ mag, which
suggest a noticeable drop in the LF amplitude from $z\simeq 6$ to $z\simeq 7$.
