Populations of quasars powered by accretion onto SMBHs with masses in excess of 10^9 solar masses are now detected when the Universe was barely a Gyr old. And in the local universe the centers of several nearby brightest cluster galaxies harbor behemoths that weigh ~ a few times 10^10 solar masses. I will discuss how these monster black holes likely form and what observational signatures can test the various formation & growth scenarios. Aside from outlining the current status of our understanding, I will close with the open exciting questions and challenges ahead for theory given the wealth of observational data in hand and in the pipeline.