There are many factors which may interfere the composition of gut microbiota in patients with CKD, such as dietary patterns, medications, uremic-related complications and comorbidities. The disturbance of normal gut microbiota, commonly called gut dysbiosis, may contribute to morbidity and mortality in this population. The putative mechanisms underlying gut dysbiosis and poor prognosis in CKD may include reduced short-chain fatty acid-producing bacteria, endotoxemia and gut-derived uremic toxins. Indoxyl sulfate is one of the most studies gut-derived uremic toxins. In this lecture, I would like to present the determinants of indoxyl sulfate in patients with CKD as well as in the healthy individuals.