This review discusses the molecular mechanisms regulating vascular endothelial permeability. Endothelial cells form a selective barrier that controls the exchange of fluids and solutes between blood and tissues. During inflammation, vascular permeability increases, allowing the passage of intravascular components into tissues. Dysregulation of endothelial permeability is linked to various diseases, including edema, cancer, and atherosclerosis. The review outlines the molecular mechanisms that regulate endothelial barrier function and physiological permeability. Endothelial barrier function is regulated by the transcellular and paracellular pathways. The transcellular pathway involves vesicle-based transport, while the paracellular pathway is controlled by junctional complexes such as tight and adherens junctions. Adherens junctions are primarily composed of VE-cadherin, which plays a critical role in maintaining endothelial integrity. Tight junctions consist of claudins, occludins, and other adhesion molecules, and their composition and number influence endothelial permeability. Small GTPases, such as RhoA, Rac1, and Rap1, regulate actin dynamics and endothelial barrier function. Vascular endothelial growth factor (VEGF) induces vascular permeability by promoting VE-cadherin phosphorylation and disrupting adherens junctions. Angiopoietins, such as angiopoietin-1 and angiopoietin-2, also regulate vascular permeability by modulating RhoA and Rac1 activities. Inflammatory mediators like histamine and thrombin induce transient opening of the endothelial barrier by activating GPCRs and altering actin dynamics. Fluid shear stress influences endothelial barrier function, with stable flow promoting barrier integrity and disturbed flow leading to dysfunction. Endothelial dysfunction and increased permeability are associated with various diseases, including chronic inflammation, cancer, and atherosclerosis. Understanding the mechanisms controlling vascular permeability is essential for developing therapeutic strategies for these diseases.This review discusses the molecular mechanisms regulating vascular endothelial permeability. Endothelial cells form a selective barrier that controls the exchange of fluids and solutes between blood and tissues. During inflammation, vascular permeability increases, allowing the passage of intravascular components into tissues. Dysregulation of endothelial permeability is linked to various diseases, including edema, cancer, and atherosclerosis. The review outlines the molecular mechanisms that regulate endothelial barrier function and physiological permeability. Endothelial barrier function is regulated by the transcellular and paracellular pathways. The transcellular pathway involves vesicle-based transport, while the paracellular pathway is controlled by junctional complexes such as tight and adherens junctions. Adherens junctions are primarily composed of VE-cadherin, which plays a critical role in maintaining endothelial integrity. Tight junctions consist of claudins, occludins, and other adhesion molecules, and their composition and number influence endothelial permeability. Small GTPases, such as RhoA, Rac1, and Rap1, regulate actin dynamics and endothelial barrier function. Vascular endothelial growth factor (VEGF) induces vascular permeability by promoting VE-cadherin phosphorylation and disrupting adherens junctions. Angiopoietins, such as angiopoietin-1 and angiopoietin-2, also regulate vascular permeability by modulating RhoA and Rac1 activities. Inflammatory mediators like histamine and thrombin induce transient opening of the endothelial barrier by activating GPCRs and altering actin dynamics. Fluid shear stress influences endothelial barrier function, with stable flow promoting barrier integrity and disturbed flow leading to dysfunction. Endothelial dysfunction and increased permeability are associated with various diseases, including chronic inflammation, cancer, and atherosclerosis. Understanding the mechanisms controlling vascular permeability is essential for developing therapeutic strategies for these diseases.