The study of the angular and spatial structure of the X-ray sky has been under investigation since the times of the Einstein X-ray Observatory. This topic has fascinated more than two generations of scientists and slowly unveiled an unexpected scenario regarding the consequences of the angular and spatial distribution of X-ray sources. It was first established from the clustering of sources making the CXB that the source spatial distribution resembles that of optical QSO. It then became evident that the distribution of X-ray AGN in the Universe was strongly reflecting that of Dark Matter. In particular, one of the key results is that X-ray AGNs are hosted by dark matter halos of mass similar to that of galaxy groups. This result, together with model predictions, has lead to the hypothesis that galaxy mergers may constitute the main AGN-triggering mechanism. However, detailed analysis of observational data, acquired with modern telescopes, and the use of the new halo occupation formalism has revealed that the triggering of an AGN could also be attributed to phenomena-like tidal disruption or disk instability and to galaxy evolution. This paper reviews results from 1988 to 2011 in the field of X-ray-selected AGN clustering. 1. Introduction After about 50 years from the opening of the X-ray window on the Universe with the discovery of Sco-X1 and the Cosmic X-ray background (CXB, [1]), our knowledge of high-energy processes in the Universe has dramatically improved. One of the leading mechanisms for the production of X-ray in the Universe is accretion onto compact objects. For this reason, the study of astrophysical X-ray sources is a powerful tool for studying matter under the effects of extreme gravity. As the efficiency of converting matter into energy in accretion processes is proportional to the “compactness” of the object (i.e., M/R), it is clear that the strongest sources powered by accretion are super-massive black holes (SMBH). It also became a cornerstone of astrophysics that every galaxy with a bulge-like component hosts a SMBH at its centre and that the BH mass and the bulge velocity dispersion are strictly related [2]. It is also believed that black holes reach those high masses via one or more phases of intense accretion activity and therefore shining as active galactic nuclei (AGN). It is believed that an AGN basically shines mostly from the power emitted by a thin, viscous, accretion disk orbiting the central SMBH Shakura and Sunyaev [3]. Such a disk produces a high amount of X-rays both from its hot inner regions (as far as the soft X-ray
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