I will review our current state of knowledge of Extreme X-ray / Optical Sources (EXO's), a population of AGN recently revealed in large surveys with Chandra, XMM, HST, Spitzer and ground-based 8-10m telescopes. These sources lie at the extreme end of the Fx/Fopt parameter space, being well detected in X-rays but faint or often undetected in optical and even near-IR bands. Their recent Spitzer detections have shed more light on their nature, and combined with the optical and near-IR data are enabling SED fits to elucidate their properties. A number of the EXO's are fit by early-type populations at redshifts ~ 2 - 5, suggesting that evolved systems are already in place at these early epochs and can contain accreting supermassive black holes. The remaining EXO's require substantial dust-reddening at redshifts ~ 3 - 6 or above, and are likely associated with recent or on-going star-formation. Generally, the EXO host galaxies appear intrinsically underluminous relative to their X-ray emission -- either their hosts are still in the process of assembling, or otherwise their central black holes have a relatively high mass or accretion efficiency. Thus, these objects provide a unique means of probing the epochs during which the black-hole / bulge mass relation may become established in the early universe.