My research interests address the questions of how galaxies form, evolve, and interact with their intergalactic environments over cosmic time. My group uses telescopes both on Earth and in space to collect images and spectra of extremely distant galaxies. Recently, we performed a large survey of the rest-frame optical spectra of a statistical sample of distant galaxies using the MOSFIRE near-infrared spectrograph on the Keck telescope atop Maunakea in Hawaii. These data offer unprecedented constraints on the stellar, gaseous, and dust contents of galaxies forming in the universe 10 billion years ago, and our group has focused in particular on how the abundance of oxygen in early galaxies provides clues to their formation. We are currently leading a large program of Hubble Space Telescope observations to constrain the contribution of star-forming galaxies to cosmic Reionization.