The complex processes that underlie normal nervous system development are known to be extremely vulnerable to perturbation by chemicals that are present in the human environment, either naturally or as a result of human activities. These processes include neurogenesis, differentiation and migration of neurons, myelination, and synaptogenesis. Children are generally at greater risk than adults of suffering adversities from chemical exposures because of their physiology and behavior. As a result, reductions in cognitive function, including intelligence, are among the most important effects of such exposures. This chapter surveys the harmful impacts on children’s brains and cognition of certain chemicals and chemical classes, including mercury, lead, organophosphate pesticides, air pollution, synthetic organic compounds (e.g., flame retardants, plastics), and compounds that disrupt the endocrine system. The final section illustrates how an exposure that causes relatively modest cognitive morbidity in an individual can nevertheless, if highly prevalent as many chemical exposures are, contribute substantially to the burden of disease at the population level.