We have assembled large samples of galaxies at redshift z ~ 4, 5 and 6 (totalling >4300 objects, >1000 objects, >500 objects, respectively) from all the deep HST ACS and NICMOS data taken to date (over 2000 orbits of data). From these we have derived rest-frame UV luminosity functions, luminosity densities, and star formation rates in a very robust and consistent way to very faint luminosities (0.01L* to 0.04L*). The faint-end slopes α of these luminosity functions are remarkably uniform and steep (α ~ −1.7), indicating very little evolution from z ~ 6 to z ~ 4. The characteristic luminosity L* brightens considerably (by ~1 mag) over this period, but the overall change in the luminosity function is such as to lead to little change in the luminosity density and star formation rate over this time. We also have detected galaxies at z ~ 7 − 8 and set strong limits at z ~ 10 directly from deep HST NICMOS observations. Spitzer observations of these z ~ 7 galaxies have been used to estimate masses and ages, suggesting substantial formation at z ~ 10 or earlier. These results show that this hierachical build-up continues into the reionization epoch.