One of the biggest challenging diseases are the neurodegenerative diseases which are not easy to target due to the presence of a complex semipermeable, dynamic, and adaptable barrier between the central nervous system (CNS) and the systemic circulation termed as the blood-brain barrier (BBB), which controls the exchange of molecules. Its semipermeable nature restricts the movement of bigger molecules, like drugs, across it and leads to minimal bioavailability of drugs in the CNS. This poses the biggest shortcoming in the development of therapeutics for CNS disorders. Although the complexity of the BBB muddles the drug delivery approaches into the CNS and can promote disease progression, understanding the composition and functions of BBB provides a platform for unraveling the way toward drug development. The BBB is comprised of brain microvascular endothelial CNS cells which communicate with other CNS cells (astrocytes, pericytes) and behave according to the state of the CNS, by retorting against pathological environments and modulating disease progression. This chapter discusses the fundamentals of BBB, permeation mechanisms, an overview of different in vitro BBB models with their advantages and disadvantages, and rationale of selecting penetration prediction methods toward the important role in the development of CNS therapeutics.