Active Galactic Nucleus (AGN) is the very small central region of an active galaxy, showing intense emissions in the high energy portions of the electromagnetic spectrum. Active galaxies also have special structural features including accretion disks, intense jets and a black hole as the most likely central engine. Detailed spectral shape of the high energy emission is studied through data obtained by both space-borne and ground based observations. Their analysis play a key role in the understanding of AGNs. In many ways, they are also considered as special laboratories for extreme physics. They can be observed at significant redshifts where the Universe was only a fraction of its known age. This book aims at modeling AGNs and their spectra in the FGST's Large Area Telescope (LAT) energy range of 20 MeV to 300 MeV. LAT data are exploited to build a phenomenological model of medium-to-high redshift AGNs. As a general trend, these objects show, at LAT energies, a bent spectrum as a result of the absorption by the Extragalactic Background Light(EBL). We specifically dealt with the analysis of Fermi-LAT data for AGNs, 3C454.3 and B21520+31 for understanding the nature of these objects.