New neurons are continuously generated in only a few areas of the adult mammalian brain--the olfactory bulb and the dentate gyrus--where they integrate into already functioning circuitry. In these regions, adult neurogenesis adds another dimension of plasticity that complements the classical molecular and cellular mechanisms of plasticity. In this talk, I will present some recent work on the functional contribution of adult-born neurons in learning and memory and then describe a new player in the regulation of adult neurogenesis : the gut microbiota.