  The unique high-resolution wide-field imaging capabilities of HST with ACS
have allowed the characterization of galaxies at redshift 6, less than 1 Gyr
from recombination. The dropout technique, applied to deep ACS i, z images in
the RCDS 1252-2927, GOODS and UDF-Parallel fields has yielded large samples of
these objects, allowing a detailed determination of their properties (e.g.,
size, color) and meaningful comparisons against lower redshift dropout samples.
The use of cloning techniques has enabled us to control for many of the strong
selection biases that affect the study of high redshift populations. A clear
trend of size with redshift has been identified, and its impact on the
luminosity density and star formation rate can be estimated. There is a
significant, though modest, decrease in the star formation rate from redshifts
z~2.5 out through z~6. The latest data also allow for the first robust
determination of the luminosity function at z~6.
