The biochemistry of bilirubin is reviewed with particular reference to newborn infants. The formation, properties, and metabolism of bilirubin are summarized and the importance of molecular shape, hydrogen-bonding, and polarity on the biologic disposition of bilirubin is emphasized. The chemical basis for the subtle influence of visible (blue) light on bilirubin structure and metabolism is explained, and recent concepts of the mechanism of phototherapy are presented. A glossary of current jargon is appended.