There are thousands of unique neurons and many types of glia in the insect central nervous system. How is this cell diversity generated? Neurogenesis begins with the delamination and enlargement of individual cells of the ventral ectoderm to form a stereotyped array of neuroblasts. Every neuroblast divides asymmetrically to generate a chain of approximately 10 smaller progeny, each of which produces a pair of neurons. Ablation, transplantation and in vitro culture experiments illuminate the role of cell interactions and cell lineage during neurogenesis, and genetic approaches in Drosophila are beginning to provide insight into the molecular mechanisms controlling these events.