A few hundred million years after the Big Bang, the first generations of stars, galaxies, and active galactic nuclei began to ionize and reheat the cold, dark, and neutral Universe. This special cosmic epoch, known as the Epoch of Reionization (EoR), holds crucial clues to how the first luminous sources evolved and how the earliest cosmic structures assembled. JWST, particularly through its wide-field slit-less spectroscopy (WFSS) capabilities, offers an unprecedented opportunity to connect individual galaxies and black holes to their dark matter halos and the surrounding cosmic web. By obtaining spectra of nearly every source within a field of view, WFSS enables efficient spectroscopic mapping of the three-dimensional early Universe.

In this talk, I will highlight recent discoveries from several treasury JWST/NIRCam WFSS surveys, focusing on three interconnected science themes: the growth of early supermassive black holes and their connection to dark matter halos, including the newly discovered population of little red dots; the emergence and assembly of the cosmic web; and the connection between AGNs, galaxies, and cosmic re-ionization. These observations are transforming our understanding of the early Universe, moving beyond individual galaxy discoveries toward statistical spectroscopic samples, three-dimensional maps of the early structures, and measurements linking
galaxies to the ionization state of the intergalactic medium.

Finally, I will briefly discuss how the Nancy Grace Roman Space Telescope will build on these JWST discoveries. By extending spectroscopic surveys to cosmological volumes, Roman will enable a new generation of statistical studies of black hole growth, galaxy evolution, large-scale structure, and re-ionization. Together, JWST and Roman are launching an exciting new era of four-dimensional exploration of the early Universe: mapping its three-dimensional structure and evolution across cosmic time.