Intestinal permeability is the property that allows solute and fluid exchange between intestinal lumen and intestinal mucosa. Many factors could have major impact on its regulation, including gut microbiota, mucus layer, epithelial cell integrity, epithelial junction, immune responses, intestinal vasculature, and intestinal motility. Any change among these factors could have an impact on intestinal homeostasis and gut permeability. Healthy condition is associated to normal intestinal permeability whereas several intestinal and extra intestinal disease, like inflammatory bowel disease, irritable bowel syndrome, non-alcoholic fatty liver disease among others, are associated to increased intestinal permeability. This review aims to synthesize determinants on intestinal permeability and to report methodologies useful to the measurement of intestinal permeability in clinical practice as well as in research settings.