Extreme X-ray/Optical ratio sources (EXOs) are a class of source that are robustly detected in our Chandra Deep Field X-ray data, while being completely undetected in our deep multi-band GOODS HST/ACS data, including the long-wavelength z-band data, yet are detected in the deep GOODS near-IR VLT imaging. These sources have values of Fx/Fopt that are factors of 10–100 times higher than those generally found for other AGN. We thus infer two plausible scenarios: (1) if these sources lie at moderately high redshifts (z ~ 3 – 5) then their hosts need to be exceedingly underluminous and/or excessively reddened compared with other AGN hosts; (2) if they lie above z ~ 6 – 7, such that their Lyman-alpha emission is redshifted out of the z-band filter, then their K-band and X-ray properties can be accounted for in terms of relatively normal hosts and moderate-luminosity AGN. In this case, these objects can serve as an invaluable probe of black hole growth and accretion activity in the early universe.