This chapter focuses on the links between the physiology of bilirubin and the multiple forms of jaundice. For the hepatologist, jaundice is the most common and obvious sign of disordered bilirubin metabolism. The two main pathophysiological consequences of bilirubin transport in the plasma are related to drug disposition and bilirubin encephalopathy (or kernicterus). Within the hepatocyte, bilirubin undergoes conjugation as a prerequisite for its biliary secretion. Conjugated bilirubin undergoes canilicular secretion. In man, as in most mammals, biliary bilirubin pigments are almost exclusively in conjugated form. It is uncertain whether the small fraction of unconjugated bilirubin is excreted as such or results from deconjugation. Biliary secretion of rubins in vivo has been investigated nearly exclusively with administration of unconjugated bilirubin. Neonatal unconjugated hyperbilirubinemia is a physiological phenomenon resulting from an increased bilirubin load and a decreased hepatic excretion mainly due to insufficient conjugation. Unconjugated hyperbilirubinemia is frequently due to overproduction caused by hemolysis or by dyserythropoesis.