  The Chandra Deep Fields (CDFs), being a major thrust among extragalactic
X-ray surveys and complemented effectively by multiwavelength observations,
have critically contributed to our dramatically improved characterization of
the 0.5-8 keV cosmic X-ray background sources, the vast majority of which are
distant active galactic nuclei (AGNs) and starburst and normal galaxies. In
this review, I highlight some recent key observational results, mostly from the
CDFs, on the AGN demography, the interactions between AGNs and their host
galaxies, the evolution of non-active galaxy X-ray emission, and the census of
X-ray galaxy groups and clusters through cosmic time, after providing the
necessary background information. I then conclude by summarizing some
significant open questions and discussing future prospects for moving forward.
