The organized arrangement of neurons in the mature spinal cord arises from a pattern of cell types that is established in the embryonic neural tube. Initial research on the molecular mechanisms that underlie this cellular diversity focused on the specification of ventral cell types, but recently more has been learned about cell-type specification in the dorsal neural tube. Genetic loss-of-function analysis in the mouse has provided important insights into the functions of several genes that direct neural cell fate, and we are beginning to define how the organization and connectivity of these neurons is established.