The resolved and unresolved components of the Isotropic Gamma-ray Background

Relying on more than 5 years of data, the Fermi Large Area Telescope (LAT) has provided a new measurement of the isotropic gamma-ray background (IGRB) up to 800 GeV. In this talk I will review our current understanding of the generation of the IGRB focusing in particular on the blazar source class that comprises the majority of sources detected by the LAT. Using a detailed and improved modeling of the blazar spectra and of their evolution we estimate the broad band, 0.1--800 GeV, contribution of blazars to the IGRB. We find that thanks to their hard spectra and large local space density BL Lacertae objects provide a substantial contribution to the IGRB at high energy (i.e. above 10 GeV). This energy range is particularly interesting because both the extragalactic background light and the intergalactic magnetic field might play an important role. Finally we will also present a refined measurement of the contribution of radio galaxies and discuss the origin of any unexplained component of the IGRB.