Tight junctions (TJ) are cell adhesion structures or junction structures that are located at the uppermost portion of the adjacent epithelial or endothelial cells in organs and tissues. TJs comprise a branched network of sealing strands, where every strand acts independently. The main function of TJs is maintaining the relative stability of intracellular substances. It functions by closing or opening the intercellular pathways and by coordinating the entry and exit of molecules of different sizes and charges. TJs are also responsible for regulating the permeability of the paracellular barrier, preventing microbial invasion, regulating cell proliferation, and maintaining cell polarity. TJ proteins connect endothelial and epithelial cells, and are also responsible for creating specific barriers and channels for paracellular transport. This book contains some path-breaking studies related to barriers and channels formed by tight junction proteins. It will serve as a valuable source of reference for graduate and postgraduate students.