Photometric studies of nearly 1000 large galaxies out to redshift z=5,
imaged by the Hubble Space Telescope in the Ultra Deep Field, reveal details
of galaxy evolution. These distances probe ages less than 2 billion years
after the Big Bang. Galaxies in the early Universe include unusual
morphologies not seen locally, such as chains, clump clusters, and tadpoles,
in addition to spirals and ellipticals. They contain star-forming regions
nearly 100 million times the mass of our sun, much more massive than those
in nearby galaxies. Average young galaxy sizes are only half that of local
galaxies, implying that disks must grow with time. Results will be presented
in the context of hierarchical build-up models.