The magnitude of the problem of neurodevelopmental disorders is enormous. Frequently, the mechanism of injury is unknown. In this article, several common developmental neurotoxins are discussed, and the function of one cell adhesion molecule, L1, will be reviewed to illustrate the principles of developmental neurotoxicology. L1 is critical for proper central nervous system development. Similarities between patients with fetal alcohol syndrome and with L1 mutations suggest that the mechanism of developmental neurotoxicity of ethanol is partly due to effects on L1 cell adhesion molecule.