Recent developments in the search for Compton-thick AGN
  I present a review of X-ray and mid-IR surveys for Compton-thick Active
Galactic Nuclei (AGN). These are the most highly obscured sources having
hydrogen column densities >1.5x10^24 cm-2. Key surveys in the local Universe
are presented including the high energy SWIFT/BAT and INTEGRAL surveys, mid-IR
and also optical surveys. Recently, deep X-ray surveys with Chandra and
XMM-Newton have produced a number of candidate Compton-thick AGN at higher
redshift primarily in the Chandra Deep Field South region. In addition, mid-IR
surveys with Spitzer have helped to develop novel complementary techniques for
the selection of Compton-thick AGN. The mid-IR techniques used to identify
Compton-thick AGN include: a) 24 micron excess sources relative to their
optical emission b) Spitzer spectroscopy for the detection of high optical
depth Si 9.7 micron absorption features c) low X-ray to 6 micron luminosity
ratio.
