An updated review of the existing knowledge regarding uremic toxins facilitates the design of experimental studies. We performed a literature search and found 621 articles about uremic toxicity published after a 2003 review of this topic. Eighty-seven records provided serum or blood measurements of one or more solutes in patients with CKD. These records described 32 previously known uremic toxins and 56 newly reported solutes. The articles most frequently reported concentrations of b2-microglobulin, indoxyl sulfate, homocysteine, uric acid, and parathyroid hormone. We found most solutes (59%) in only one report. Compared with previous results, more recent articles reported higher uremic concentrations of many solutes, including carboxymethyllysine, cystatin C, and parathyroid hormone. However, five solutes had uremic concentrations less than 10% of the originally reported values. Furthermore, the uremic concentrations of four solutes did not exceed their respective normal concentrations, although they had been previously described as uremic retention solutes. In summary, this review extends the classification of uremic retention solutes and their normal and uremic concentrations, and it should aid the design of experiments to study the biologic effects of these solutes in CKD. The uremic syndrome is characterized by the retention of various solutes that would normally be excreted by the kidneys. The substances that interact negatively with biologic functions are called uremic toxins. In the past years, research on uremic toxicity has been very dynamic and resulted in the identification of dozens of retention solutes, including several uremic toxins. In 2003, the European Uremic Toxin Work Group (http://www.uremic-toxins.org/) proposed a classification of 90 retention solutes providing data on normal and pathologic serum concentrations. 1 In 2007, results were further discussed and expanded with the addition of 14 solutes. 2,3 This collaborative work focused on the highest mean or median concentration of the solutes measured in a uremic population and the highest individual uremic concentration. These data were particularly relevant for researchers on uremic toxicity, and they became a successful tool for allowing use of standardized and biologically relevant concentrations in experimental settings. More recently, scientific and technological progress resulted in the identification of many new uremic retention solutes, particularly thanks to nontargeted approaches such as metabolomic and proteomic profiling. 4,5 To maintain experimental guidelines in keeping with current knowledge, it seemed necessary to propose an update of the encyclopedic review.