Developmental neurotoxicology is the study of adverse effects to the nervous system resulting from exposures during development. Because of the protracted nature of nervous system development this can encompass toxic insults in utero through early adolescence. The results of these exposures are complex and varied, they can range in severity from severe lesions to subtle changes in gene expression that produce changes in function or behavior. This chapter will present an overview of nervous system development and will provide examples of how toxic exposures can interrupt the normal developmental processes in the CNS, producing a wide variety of pathologies. Additionally this chapter will explore other facets of the developing nervous system that make it uniquely susceptible to toxic insult.