  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
how the source properties change with redshift. This article reviews X-ray
observations of AGN from redshifts of ~0.1-3 with 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.
