Pentosidine is a highly fluorescent advanced glycation end product (AGE) and crosslink derived from one molecule of arginine and one of lysine bridged in an imidazo-pyridinium structure (Fig. 1). It was first isolated from articular cartilage by Sell and Monnier (1), and has now been detected and quantified in a variety of human and animal tissues, including skin and kidney collagen (2–5), lens crystallins (6, 7), plasma (8, 9), serum (10), urine (11), and synovial fluid (12, 13). Pentosidine is readily prepared from arginine, lysine, and a pentose (hence its name). Dyer et al. (14) have also described its formation from glucose, albeit at a slower rate and probably through oxidation of glucose to arabinose (15). Because its formation from either glucose or ribose requires oxidation, pentosidine is both an AGE and a “glycoxidation” product (16).