Figure 17: Space density of Compton thick AGN as a function of redshift, as published by Treister et al. [75]. Filled triangles show the ECDF-S results [75]. Squares: X-ray-selected sources in the CDF-S [70]. Star: Measurement obtained using mid-IR spectroscopy [77]. Pentagons: Values obtained using mid-IR excess sources in COSMOS [78]. Solid lines show the expected space density of Compton thick AGN from the luminosity function of Yencho et al. [79], with the overall normalization fixed to the results of the INTEGRAL and Swift/BAT surveys [35], while the dashed lines show the expectations based on the luminosity function of Della Ceca et al. [80]. Red symbols show measurements and expectations for  erg s−1 sources, while the blue symbols are for  erg s−1. While for the lower luminosity sources a good agreement is found between observations and expectations, higher luminosity sources at lie well above the luminosity function.