To obtain the correct number of each neuronal subtype, there must be mechanisms to control progenitor pool size, and factors that control the differentiation of these progenitors into specific types of neurons. In the dorsal spinal cord, recent advances have begun to define these mechanisms. Eight dorsal interneuron populations have now been classified according to their expression of molecular markers, their projection patterns, neurotransmitter type, and/or function. The ability to identify progenitor cells and neurons in the dorsal spinal cord on the basis of the genes they express has provided a framework for identifying extrinsic factors that establish proliferation rate and dorsal-ventral polarity in the developing neural tube; furthermore, this ability has helped define roles for basic helix-loop-helix and homeodomain transcription factors in neuronal cell-type specification.