The blood-brain barrier (BBB) is a cellular barrier that comprises specialised brain endothelial cells that form the walls of microcapillaries and separates the blood from the brain interstitial fluid. The BBB can regulate the passage of endogenous substances and xenobiotics into the brain. This maintains brain homeostasis and neuronal signalling. The barrier is not static and can be considered a biologically active interface with regulatory actions involving transport, secretory and enzymatic roles.