The Decadal Survey on Astronomy and Astrophysics (Astro2020) of the US National Academies of ScienceWe describe the opportunities for galaxy cluster science in the high- redshift regime where massive, virialized halos first formed and where star formation and AGN activity peaked. New observing facilities from radio to X-ray wavelengths, combining high spatial/spectral resolution with large collecting areas, are poised to uncover this population