International Journal of Modern Physics D c © World Scientific Publishing Company 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> 1.5 × 1024 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. Re-cently, 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 tech-niques used to identify Compton-thick AGN include: a) 24 µm excess sources relative to their optical emission b) Spitzer spectroscopy for the detection of high optical depth Si 9.7µm absorption features c) low X-ray to 6µm luminosity ratio