The Contribution of BL Lacertae Objects to the Isotropic Gamma-ray Background

BL Lacertae (BL Lac) objects represent one of the most interesting source populations in the gamma-ray sky. Contrary to flat-spectrum radio quasars and even misaligned radio galaxies, the number of BL Lac objects detected by Fermi in its "clean" samples is dramatically increasing, going from 275 objects detected in the first year to 395 in the second year. They include sources detected with a hard spectrum up to a redshift of ~2. This population can thus provide a substantial contribution to the intensity of the Isotropic Gamma-ray Background (IGRB) at the highest (i.e., >10 GeV) energies. In this talk, I will review the properties of BL Lac objects at gamma-ray energies and current efforts to constrain their redshifts. I will also present the first luminosity function of gamma-ray selected BL Lac objects and an estimate of the contribution of this population to the IGRB. Implications for the formation of the >10 GeV background will also be discussed.