The cores of active galactic nuclei (AGN) harbor some of the most extreme conditions of matter and energy in the Universe. One of the major goals of high-energy astrophysics is to probe these extreme environments in the vicinity of supermassive black holes, which are intimately linked to the mechanisms that produce the continuum emission in AGN. X-ray studies seek to understand the physics responsible for the continuum emission, its point of origin, how nuclear activity is fueled, and how supermassive black holes evolve. The key to finding answers to these questions lies in measuring the intrinsic luminosities and spectral shapes, the relation of these properties to other wavebands, and now the source properties change with redshift. This article reviews X-ray observations of AGN from redshifts of∼0.1−3with the goal of summarizing our current knowledge of their X-ray spectral characteristics. Results are evaluated in terms of their robustness and are examined in the light of current theoretical predictions of energy release via processes associated with the accretion mechanism. A possible evolutionary scenario is discussed, along with the importance of AGN studies at high redshift as they relate to the total energetics of the Universe. ©1999 American Institute of Physics.