Even though the first population of star-forming galaxies at z > 6 have been discovered in the deepest imaging campaigns with the Hubble Space Telescope, the redshift and intrinsic faintness of these objects currently present a challenge for spectroscopy. As such, relatively little is known about the physical conditions in galaxies at the earliest cosmic times and the limited observations we do have pose challenges to current stellar population models. We can, however, make progress on these issues with spectroscopic observations of “extreme” galaxies at lower redshifts (1 < z < 6). I will highlight my recent work in this direction from cutting edge facilities such as MUSE on the VLT as well as ultra-deep HST imaging and spectroscopy. In one particular region, the Hubble Ultra Deep Field, I have used these two facilities to construct a census of star-forming galaxies to unprecedented depths. This includes an abundant population of galaxies with physical properties similar to those at the highest redshifts, and a hitherto invisible population that represent the faintest and lowest-mass galaxies ever discovered at cosmological distances. Detailed studies of these populations serve as a preview of science in the JWST era, shedding light onto the galaxy formation process and cosmic Reionization.