Tight junctions from a morphological and functional boundary between the apical and basolateral cell surface domains of epithelia and endothelia, and regulate selective diffusion along the paracellular space. Two types of four-span transmembrane proteins, occludin and claudins, as well as the single-span protein JAM are associated with tight junctions. The functional analysis of these proteins starts to reveal how they are involved in the functions of tight junctions, which of their domains are important for these functions, and how they interact with each other to form the junctional diffusion barriers.