Abstract　

 　 　 
I briefly introduce the Chandra Deep Field-South (CDF-S) survey. With a 4 Ms exposure, it is the deepest Chandra survey ever performed; the AGN sky density in this field reaches ~10,000 deg^-2. I present our approach to reliably identify and classify the deep field X-ray sources. On the other hand, a significant fraction of the AGN population probably remains undetected even in the deepest X-ray surveys, based on studies of the cosmic X-ray background and AGN population-synthesis models. These missing AGNs are likely heavily obscured or even Compton-thick, and they are considered to be comparably numerous to the unobscured and moderately obscured AGNs. We searched for such AGNs in the CDF-S utilizing IR, X-ray, and multiwavelength data, and we revealed a numerically significant population of X-ray undetected heavily obscured AGNs.