Neocortical interneurons are very diverse in morphological, physiological, molecular, and developmental characteristics. Recent work is discovering strong correlations between these phenotypic features, confirming the intuition of Cajal and Lorente that distinct classes of interneurons exist, each presumably mediating a different circuit function. A paper by Butt et al. in this issue of Neuron describes correlations between the developmental origin of interneurons and their anatomical, electrophysiological, and molecular properties. An effort to standardize the nomenclature of interneurons is underway. Because different interneuron subtypes have different ontogenic origin, they could be classified based on their developmental specification by transcription factors.