 The blood-brain barrier is a well-controlled stable environment in adults that protects the brain from harmful substances, but it is immature or leaky in embryos and newborns, making them more vulnerable to drugs and toxins entering the fetal circulation. New evidence shows that many adult mechanisms are present in embryonic brains, but some are not, such as specific transport of plasma proteins across the blood-CSF barrier and embryo-specific intercellular junctions between neuro-apendymal cells lining the ventricles. Additionally, developing cerebral vessels appear to be more fragile than in adults, rendering developing brains more vulnerable to drugs, toxins, and pathological conditions that contribute to cerebral damage in later neurological disorders. After birth, loss of protection by e-flex transporters in the placenta may also make the neonatal brain more vulnerable than in the fetus. This article was authored by Norman Arsonders, Shane A. Liddolo, and Katarzyna M. Zygieliska.