  Mid-infrared observations of Active Galactic Nuclei (AGN) are important for
understanding of the physical conditions around the central accretion engines.
Chandra and XMM-Newton X-ray observations of a 300 arcmin^2 region in the
Extended Groth Strip are used to select a sample of~150 AGN. The Spitzer
instruments IRAC and MIPS detect 68-80% of these sources, which show a wide
range of mid-infrared properties. About 40% of the sources have red power-law
spectral energy distributions (f_nu ~ nu^alpha, alpha<0) in the 3.6-8 um IRAC
bands. In these sources the central engine dominates the emission at both X-ray
and IR wavelengths. Another 40% of the sources have blue mid-IR spectral energy
distributions (alpha>0) with their infrared emission dominated by the host
galaxy; the remaining 20% are not well-fit by a power law. Published IRAC color
criteria for AGN select most of the red sources, but only some of the blue
sources. As with all other known methods, selecting AGN with mid-IR colors will
not produce a sample that is simultaneously complete and reliable. The IRAC SED
type does not directly correspond to X-ray spectral type (hard/soft). The
mid-IR properties of X-ray-detected Lyman-break, radio, submillimeter, and
optically-faint sources vary widely and, for the most part, are not distinct
from those of the general X-ray/infrared source population. X-ray sources emit
6-11% of the integrated mid--IR light, making them significant contributors to
the cosmic infrared background.
