In chronic kidney disease there is a build-up of metabolic waste products known as uremic retention solutes, the amount of which increases as kidney function declines. Uremic retention solutes that have a toxic effect on cells and tissues are called uremic toxins, and disturbances in cells and tissues are called uremic syndrome. In CKD, there are changes in the gut microbiota where there is an increase in the growth of harmful proteolytic microbiota and a decrease in the protective saccharolytic microbiota. These changes in the gut microbiota lead to the formation of gut-derived uremic toxins, namely p-cresyl sulphate, indoxyl sulphate, trimethylamine N-oxide/TMAO, dimethylglycine and glutarate. These have been shown to lead to increased inflammation, progression of CKD, cardiovascular complications, brain damage and immune system dysfunction.