Long-duration GRBs serve as a useful reference point, both for comparison and as an instructive example of the utility of such an approach \cite{Perley2016}.  Compared both to the host galaxies of other stellar explosions (e.g., core-collapse supernovae) and (to a lesser extent) the broader population of star-forming galaxies, long-duration GRB occur in galaxies that are lower in (stellar) mass, lower in metallicity, and higher in specific star-formation rate (star formation per unit mass).  The morphology is often irregular.  Long GRBs may also prefer compact galaxies, and dense and/or central regions of galaxies, more than other types of core-collapse explosion.  They trace the UV light within their hosts more closely than other cosmic explosions.  All of these properties are consistent with a young, massive, and likely low metallicity progenitor - perhaps some of the most extreme (and rapidly rotating) massive stars in the Universe explode as long-duration GRBs (Figure 14).