Despite the existence of two barrier systems in the brain, research over the last century has mostly focused on the blood-brain barrier rather than on the blood-CSF barrier. Today, there is a greater understanding of the function of the blood-CSF barrier and of the choroid plexus, a tissue that is the primary site of this barrier. With the growing number of studies that focus on the role of the blood-CSF barrier in CNS homeostasis and neurological disorders, a modern overview of the blood-CSF barrier is long overdue. The Blood-Cerebrospinal Fluid Barrier is exclusively devoted to the blood-CSF.