Abstract : Sensitive cosmic X-ray surveys with the Chandra, XMM-Newton, and NuSTAR observatories have revolutionized our ability to find and study distant active galactic nuclei (AGNs), the main sites of supermassive black hole growth in the Universe. I will describe some of the resulting discoveries about the demographics, physics, and ecology of AGNs. Topics covered will include the utility of deep X-ray plus multiwavelength surveys for investigating distant AGNs; evolution constraints for the typical AGNs of the distant Universe; the cosmic balance of power between supermassive black holes and stars; interactions between AGNs and their hosting galaxies; and the AGN content of newly forming galaxies. I will end by discussing some key outstanding questions and new observations and missions that aim to answer them.