One of the most exciting areas of research in astronomy is early galaxy formation. Substantial progress has been made over the last few years in identifying large numbers of distant galaxies with Hubble. However, understanding the build-up of stars and characterizing their physical properties (i.e., the next logical steps) has been a challenge, due to the extreme faintness of most galaxies in the distant universe. A detailed physical study of galaxies is nonetheless possible over a region of the Hubble Ultra Deep Field containing the deepest-ever observations obtained with the Hubble and Spitzer Space telescopes. Taking full advantage of these new data to study galaxies requires sensitive spectroscopic and longer wavelength observations. Our Leiden team is obtaining the former information with the MUSE instrument and a guaranteed-time program. Our on-going spectroscopy is probing 10× fainter than is currently state-of-the- art. We anticipate a further 2× improvement leveraging the new adaptive-optics capabilities of GALACSI. Longer-wavelength observations (indicating the star-formation hidden by dust and molecular-gas fuel for star formation) are being provided by ASPECS, a new ALMA large program. Our objective is to characterize the build-up of galaxies in mass. To maximize both the accuracy and physical relevance of our new results on early galaxies with MUSE, we will conduct a parallel study of ∼1000s of lower-redshift analogues to this distant sample also available in our MUSE data set. Funding is requested for two postdoctoral fellows and a PhD student to make use of this significant opportunity to study early galaxy assembly.