Elevation of the serum bilirubin level is a common if not universal finding during the first week of life. This can be a transient phenomenon that will resolve spontaneously. Alternatively, hyperbilirubinemia may signify a serious or even potentially life-threatening condition. There are many causes of hyperbilirubinemia, and each has its own therapeutic and prognostic implications. Independent of the cause, elevated serum bilirubin levels may be potentially toxic to the newborn infant. This chapter begins with a review of perinatal bilirubin metabolism. Assessment, etiology, toxicity, and therapy for neonatal jaundice are then addressed. Finally, the diseases in which there is a primary disorder in the metabolism of bilirubin are reviewed regarding their clinical presentation, pathophysiology, diagnosis, and treatment. Other pertinent reviews have been published [1–3]. BILIRUBIN METABOLISM Production and Circulation In 1864, Städeler  used the term bilirubin, derived from Latin (bilis, “bile”; ruber, “red”), for the red-colored bile pigment. Bilirubin is formed from the degradation of heme-containing compounds (Figure 13.1). The largest source for the production of bilirubin is hemoglobin. However, other heme-containing proteins are also degraded to bilirubin, including the cytochromes, catalases, tryptophan pyrrolase, and muscle myoglobin . The formation of bilirubin is initiated by cleaving the tetrapyrrole ring of protoheme (protoporphyrin IX), which results in a linear tetrapyrrole (biliverdin). The first enzyme system involved in the formation of bilirubin is microsomal heme oxygenase (HO). Two major forms of HO have been identified . HO1, the inducible form, is located in the spleen and liver.