Source Populations in the Fermi-LAT Sky and the Origin of the Isotropic Diffuse Background

Fermi-LAT, with its unprecedented sensitivity, detected over a thousand point-like sources during the first year of operations. Since most of them are blazars, the study of their properties with Fermi-LAT represents a real breakthrough in the understanding of their nature. In this talk, the main features of the sample of sources detected by Fermi will be reviewed focusing in particular on the statistical properties of blazars. We will present the logN-logS distributions of the entire blazar class and separately for BL Lac objects and FSRQs. Their contribution to the isotropic background emission, as measured by LAT, will be evaluated. We will also discuss the cosmological evolution of FSRQs and BL Lacs in the context of growth of super-massive black holes.