 The blood-brain barrier, BBB, is a well-controlled stable environment that protects the adult brain from harmful substances in the blood and cerebrospinal fluid. However, evidence suggests that this barrier may be immature or leaky during embryonic development, making the developing brain more vulnerable to drugs and toxins. Some mechanisms present in embryos are not present in adults, such as specific transport of plasma proteins across the BBB and embryo-specific intercellular junctions between neuroependymal cells lining the ventricles. Additionally, developing cerebral vessels may be more fragile than in the adult, contributing to cerebral damage and neurological disorders. After birth, loss of protection by e-flux transporters in the placenta may also make the neonatal brain more vulnerable than in the fetus. This article was authored by Norman Arsonders, Cheney Lytillo, and Katarzyna M. Zygieliska.