As an observer he has been the PI of the “COS-Halos” project, a 277-orbit program with Hubble’s Cosmic Origins Spectrograph that has made several significant discoveries about the nature of the gaseous Circumgalactic Medium (CGM) that helps to form and feed galaxies. This program has generated a number of high-impact publications, including the most cited paper based on COS data (Tumlinson et al., Science, 334, 938). As a theorist, he was among the first to publish realistic stellar evolution models and ionizing photon outputs for primordial stars (Tumlinson & Shull 2000; Tumlinson, Venkatesan, & Shull 2003). He was an early proponent of using the fossil record of chemical abundances to constrain the first stars IMF (Tumlinson et al. 2004, 2010). This work ultimately led to a state-of-the-art Hubble imaging program to obtain the star- formation histories and IMFs of the Ultra-faint Dwarf galaxies with HST/ACS (Brown et al. 2012; Geha et al. 2013). This work has made key predictions about the first stars that will soon be tested by JWST.