The Origin of the Extragalactic Gamma-ray Background

The Fermi Large Area Telescope (LAT) has provided a new measurement of the Extragalactic Gamma-ray Background (EGB) up to ~820 GeV. This talk will review our current understanding of the generation of the EGB focusing in particular on the blazar class. Using an improved modeling of the blazar spectra and of their evolution we estimate the broad band integrated emission of blazars. Thanks to their hard spectra and large space density BL Lacertae objects provide a substantial contribution to the EGB at > 100 GeV. This energy range is particularly interesting because both the extragalactic background light and the intergalactic magnetic field might play an important role. We will also show that blazars, together with radio and star forming galaxies can account for most of the measured EGB intensity. This places competitive constraints on the cross-section for Dark Matter annihilation.