Uremic toxins are compounds which exert biological action and which are retained in the body of patients with renal failure, whereas they normally should be excreted by the healthy kidneys into the urine. Only few retention solutes conform with the strict definition of uremic toxins. Uremic solutes can be subdivided into small water soluble compounds, small protein bound compounds and middle molecules. Most small water soluble compounds are not very toxic and the toxic ones often show a kinetic behavior that is different from that of urea. The uremic solutes that play a role in inflammation and cardio-vascular complications most often are middle molecules and/or protein bound. Hence, once more, the current marker urea is not representative for their mechanistic behavior.