Dysfunction of the mucosal barrier, characterized by increased intestinal mucosal permeability, is implicated in the pathogenesis of not only local intestinal diseases but also systemic diseases, including metabolic syndrome, cardiovascular diseases, and neurodegenerative disorders. Therefore, the development and standardization of reliable methods for evaluating intestinal mucosal permeability are of great importance. This chapter has provided an overview of current methods for assessing intestinal mucosal permeability, including the short-circuit current method using the Ussing chamber, the lactulose/mannitol permeability test, and endoscopic intestinal mucosal permeability testing using a tissue conductance meter. The short-circuit current method is valuable for basic research using animal models; it is not suitable for routine clinical testing due to its requirement for multiple biopsy specimens and its procedural complexity. The lactulose/mannitol permeability test is clinically useful; however, it does not allow for distinction between the relative contribution of the small intestine and colon to overall mucosal permeability. In contrast, endoscopic intestinal mucosal permeability testing using a tissue conductance meter enables real-time evaluation of intestinal mucosal permeability in specific intestinal regions during endoscopy. Among the available methods, the tissue conductance meter, allowing in vivo, real-time, site-specific assessment during routine endoscopic procedures, is expected to gain broader application in clinical practice.