Based on a systematic PubMed search, this short review addresses why intestinal permeability may be important in the pathobiology of irritable bowel syndrome (IBS), the evidence of abnormal permeability in patients with IBS, and the pros and cons of the different probe molecules available to assess intestinal permeability. While a subgroup of patients with IBS appears to have evidence of increased intestinal permeability, improvements in the methods and validation are key to further research in this field in order to better understand intestinal barrier functions in IBS.