Blood–brain barrier is a highly selective physiological interface separating the circulating blood from the central nervous system parenchyma. As a critical research concept, it encompasses the comprehensive investigation of the specialized cellular and molecular components, including endothelial cells with tight junctions, pericytes, and astrocytes, that form this barrier, studying its dynamic regulation of molecular and cellular transport, its role in maintaining cerebral homeostasis and protection, and its significant implications for neurological disease pathogenesis and the development of central nervous system therapeutics.