Hyperbilirubinemia or jaundice refers to excessive levels of bilirubin in the serum of newborn infants. It is of interest to developmentalists, since serum bilirubin can cross the blood–brain barrier and, in high levels, may cause brain damage, particularly in the globus pallidus, substantia nigra reticulata, subthalamic nucleus, brainstem auditory structures (vestibular and cochlear), oculomotor nuclei, the hippocampus, and the cerebellum. Very high levels of bilirubin can cause the classic acute and chronic bilirubin encephalopathies. Controversy exists as to whether lower levels cause minor neurological, cognitive, or behavioral deficits. Hyperbilirubinemia develops in neonates primarily due to their physiologic immaturity, although other conditions and factors may play a role. Bilirubin is a yellow pigment that results from the breakdown of hemoglobin from red blood cells. In routine clinical practice, bilirubin is measured as total serum bilirubin (TSB). Many healthy full-term infants develop a mild degree of jaundice usually termed “physiologic” jaundice or jaundice not attributable to pathologic factors or disease. The number and rate of breakdown of red cells is higher in the newborn and leads to an increased release of bilirubin to the circulation. The newborn’s liver has reduced capacity to take up bilirubin due to immaturity. Additionally, loss of water in combination with reduced intake of fluid prior to establishment of breast feeding may make the infant jaundiced because of dehydration (Stevenson et al. 2004). Although most neonatal jaundice is physiologic, Table 33.2 lists some of the more common ‘‘pathologic’’ mechanisms causing jaundice in newborns (Stevenson et al. 2004). In actuality, all healthy, full-term infants develop some level of neonatal hyperbilirubinemia as a consequence of physiological immaturity in metabolizing bilirubin, mild dehydration, and/or factors in the breast milk (if they are breast-feeding) (Davidson 1941; Maisels et al. 1986). Clinical jaundice is visible at serum bilirubin levels of approximately 5–7 mg/dL, and approximately 50% (Palmer and Mujsce 2001) of all normal newborns appear jaundiced during the first week of life.