The existence of tight junctions between epithelial cells has been known for over 100 years, but their exact functions remained elusive until recently. The present paper summarizes the newest knowledge about morphology, molecular components and regulation of tight junctions. In the chapter about morphology the criteria for classification of epithelia as well as the importance of the "freeze fracture" electron-microscopic technique in forming our present ideas about the fine structure of tight junctions will be discussed. The two existing theories about the molecular composition of the resistance-forming barrier ("lipid-model" and "protein-model", resp.) and the important properties of the tight-junction-associated proteins characterized in the past years are discussed in the chapter about molecular components. Finally, in the last part of the paper the factors playing an important role in the regulation of tight junctions, including the cytoskeleton, the protein kinases A and C, the phospholipase C and two categories of the GTP-binding proteins are reviewed.