Tight junctions are located at the luminal aspect of adjacent epithelial cells and form a barrier that limits the paracellular diffusion of hydrophilic solutes. In recent years, evidence has accumulated to indicate that tight-junction permeability is regulated by the absorption of various nutrients. In this review, we present the physiological basis and importance of tight-junction regulation in intestinal epithelium. The molecular structure of tight junctions and their interactions with the cell cytoskeleton as well as the physical and chemical forces that influence tight junction permeability are described. Much of this review addresses the controversial Pappenheimer hypothesis, which states that a major portion of intestinal glucose absorption occurs through tight junctions and not by saturable transcellular active transport. The absorption of a significant portion of glucose through tight junctions requires increased junctional permeability, a very high intralumenal glucose concentration, and a sufficient osmotic gradient to promote volume flow.