Intestinal mucosa has an absorptive function and acts also as a selective barrier against potential antigenic, toxic and carcinogenic substances. Intestinal permeability can be defined as the capacity of mucosal surface to be penetrate by specific substances through unmediated diffusion. There are two theories about molecular permeation routes: the first one hypothesizes a transcellular (through small pores), a paracellular (through big channels) and a lipophilic pathways; the second one gives a key role only to paracellular tight-junctions. In many diseases we can find changes in intestinal permeability evaluable by simple and non invasive tests, administering "per os" probe molecules. These substances cross the epithelium in different way and amount according to their physicochemical features and mucosal integrity; then they reach circulation and are eliminated in urines where they can be detected. The most frequently molecules used are mono/disaccharides, 51Cr-labelled ethylenediaminetetraacetate (51Cr-EDTA) and polyethylene glycol (PEG). This simple method has become more and more used for diagnostic and speculative aims. These intestinal permeability tests have a low specificity so they cannot be used for a definitive diagnosis of intestinal disease; nevertheless, the high sensitivity for intestinal mucosal damage could make them a necessary method to evaluate mucosal integrity after therapy, to select patients with a specific symptoms and to support, particularly in pediatric populations, more specific and invasive diagnostic tests.