This chapter focuses on neurotoxicity of the selected group of synthetic pesticides and their association with neuropsychiatric diseases. The ontogeny of the brain and its vulnerabilities have been reviewed by several investigators [66-68]. Small body size, different ratio of fat, muscle, and water, higher breathing and metabolic rate per body weight, and immature blood-brain barrier in comparison to adults add to their further vulnerability to insults. While genetic inheritance has an important role in fetal brain development, environmental factors including exposure to pesticides, industrial chemicals, pharmaceuticals, nutrition, infectious agents, and other psychosocial indices are reported to exert profound influence on the brain development by altering the signaling in specific brain regions. Further, the presence of dieldrin in autopsied brain samples of PD cases in separate case control studies indicates that organochlorine pesticides may affect the dopaminergic neurons in the substantia nigra.