Across the animal kingdom the apical junction complex of epithelial cells creates both a permeability barrier and cell polarity. Although based on overlapping and evolutionarily conserved proteins, the cell–cell contacts of nematodes, flies and mammals appear to differ in morphology and functional organization. Emerging evidence shows that the selective pore-like properties of vertebrate and invertebrate barriers are created by the claudin family. Similarly, assembly of the barriers requires a conserved set of polarity-generating protein complexes, particularly the PAR protein complexes.