The intestinal epithelial barrier is a critical structure that regulates interactions between the intestinal lumen, immune system, and body while supporting nutrient and waste transport. It is composed of a continuous layer of epithelial cells and junctions that seal the paracellular space. Compromised barrier function is associated with various diseases, but current therapies targeting the barrier are limited. Understanding the molecular mechanisms of barrier regulation is essential for developing effective treatments. The intestinal barrier function is determined by the epithelial monolayer, which regulates the selective permeability of water, ions, and macromolecules. The mucus layer provides a protective barrier but does not control transmucosal flux. The apical junctional complex (AJC), including tight junctions, adherens junctions, and desmosomes, plays a key role in maintaining barrier function. Tight junctions are the primary regulators of paracellular permeability, with claudins being the most important transmembrane proteins. Claudin-2 expression is linked to changes in trans-epithelial resistance and barrier function. Tight junction proteins, such as ZO family proteins, interact with the actin cytoskeleton and regulate epithelial barrier function. Mutations in ZO-2 can lead to severe liver disease, highlighting the importance of tight junction proteins in homeostasis. The pore and leak pathways are distinct routes across the paracellular barrier, with the pore pathway being size- and charge-selective and the leak pathway having limited selectivity. Both pathways are regulated by various factors, including cytokines and signaling pathways. In diseases like inflammatory bowel disease (IBD) and celiac disease, barrier dysfunction is associated with increased intestinal permeability and disease severity. Experimental models have shown that targeting the epithelial barrier can reduce disease severity. However, current therapies are limited, and more research is needed to understand the mechanisms of barrier regulation and develop effective treatments. Targeting the epithelial barrier, such as through MLCK inhibition or claudin-2 modulation, shows promise but requires further investigation. Overall, the intestinal barrier is a critical component of intestinal health, and its dysfunction contributes to various diseases.The intestinal epithelial barrier is a critical structure that regulates interactions between the intestinal lumen, immune system, and body while supporting nutrient and waste transport. It is composed of a continuous layer of epithelial cells and junctions that seal the paracellular space. Compromised barrier function is associated with various diseases, but current therapies targeting the barrier are limited. Understanding the molecular mechanisms of barrier regulation is essential for developing effective treatments. The intestinal barrier function is determined by the epithelial monolayer, which regulates the selective permeability of water, ions, and macromolecules. The mucus layer provides a protective barrier but does not control transmucosal flux. The apical junctional complex (AJC), including tight junctions, adherens junctions, and desmosomes, plays a key role in maintaining barrier function. Tight junctions are the primary regulators of paracellular permeability, with claudins being the most important transmembrane proteins. Claudin-2 expression is linked to changes in trans-epithelial resistance and barrier function. Tight junction proteins, such as ZO family proteins, interact with the actin cytoskeleton and regulate epithelial barrier function. Mutations in ZO-2 can lead to severe liver disease, highlighting the importance of tight junction proteins in homeostasis. The pore and leak pathways are distinct routes across the paracellular barrier, with the pore pathway being size- and charge-selective and the leak pathway having limited selectivity. Both pathways are regulated by various factors, including cytokines and signaling pathways. In diseases like inflammatory bowel disease (IBD) and celiac disease, barrier dysfunction is associated with increased intestinal permeability and disease severity. Experimental models have shown that targeting the epithelial barrier can reduce disease severity. However, current therapies are limited, and more research is needed to understand the mechanisms of barrier regulation and develop effective treatments. Targeting the epithelial barrier, such as through MLCK inhibition or claudin-2 modulation, shows promise but requires further investigation. Overall, the intestinal barrier is a critical component of intestinal health, and its dysfunction contributes to various diseases.