Olfactory bulb (OB) interneurons are continuously generated throughout development and in adulthood, and are derived from different progenitor zones. Once integrated in the OB circuits, interneurons play essential roles in olfactory information processing by modulating the activity of major output neurons. These functions are performed by multiple classes of neurons that differ in their spatial distribution, morphology, neurochemical and synaptic properties. This diversity, and the continuous neurogenesis make the understanding of the specification mechanisms in the OB a challenging task. New studies suggest that both intrinsic and extrinsic cues are involved in fate determination of OB interneurons. In both development and adulthood the expression of specific transcription factors not only defines different progenitor regions but also precise interneuronal phenotypes. Here we discuss recent findings on the molecular mechanisms regulating production and diversity of OB interneurons with respect to the spatial and temporal parameters.