New science shows that exposure to toxins prenatally or early in life can have a devastating and lifelong effect on the developing architecture of the brain. Exposures to many chemicals have much more severe consequences for embryos, fetuses, and young children, whose brains are still developing, than for adults. Substances that can have a truly poisonous effect on the brain--known as neurotoxins--can be found in environmental chemicals such as lead and mercury, in recreational drugs such as alcohol, nicotine, and cocaine, and in prescription medications, such as some acne treatments. Most neurotoxin exposure is preventable. This report summarizes the complex scientific research on which toxins present the greatest risk at various stages of brain development, addresses popular misconceptions about the relative risk and safety of some common substances, and suggests policies that can help reduce the enormous human and economic costs of exposure to toxins during development.