The predominant function of both epithelial and endothelial cell layers is to build up a highly selective permeability barrier between two fluid compartments which is of immense physiological importance as it abolishes uncontrolled permeation of solutes and metabolites from one compartment to the other. The structural basis for these barrier properties are very special cell-to-cell contacts, the so called tight junctions that occlude the intercellular clefts and therefore, avoid free diffusion of hydrophilic compounds via the paracellular shunts. Up to now the molecular composition of the tight junctions is not known in detail. All experimental findings collected so far do not allow to decide unequivocally whether tight junctions are basically proteinaceous or lipidic in nature. The present review is intended to summarize recent findings about junctional composition, emphasizing the potential role of non-lamellar lipid structures in the formation of tight junctions.