The blood–brain barrier (BBB) is a unique property of the microvasculature in the central nervous system (CNS), which allows for the precise regulation of ion and molecule movement between the blood and the brain. This barrier is crucial for maintaining proper neuronal function and protecting the CNS from toxins, pathogens, and injury. The BBB is composed of endothelial cells (ECs) that form tight junctions, express specific transporters, and have low transcytosis rates. These properties are regulated by interactions with mural cells (pericytes and vascular smooth muscle cells), astrocytes, and immune cells. The BBB's integrity is essential for preventing immune cell infiltration and maintaining homeostasis in the CNS. Disruption of the BBB, often observed in neurological disorders such as multiple sclerosis (MS), stroke, and Alzheimer's disease, can lead to edema, ionic imbalance, and neuronal degeneration. Understanding the molecular and cellular mechanisms that regulate the BBB is crucial for developing therapeutic strategies to modulate or bypass the barrier for drug delivery to the CNS.The blood–brain barrier (BBB) is a unique property of the microvasculature in the central nervous system (CNS), which allows for the precise regulation of ion and molecule movement between the blood and the brain. This barrier is crucial for maintaining proper neuronal function and protecting the CNS from toxins, pathogens, and injury. The BBB is composed of endothelial cells (ECs) that form tight junctions, express specific transporters, and have low transcytosis rates. These properties are regulated by interactions with mural cells (pericytes and vascular smooth muscle cells), astrocytes, and immune cells. The BBB's integrity is essential for preventing immune cell infiltration and maintaining homeostasis in the CNS. Disruption of the BBB, often observed in neurological disorders such as multiple sclerosis (MS), stroke, and Alzheimer's disease, can lead to edema, ionic imbalance, and neuronal degeneration. Understanding the molecular and cellular mechanisms that regulate the BBB is crucial for developing therapeutic strategies to modulate or bypass the barrier for drug delivery to the CNS.