Biliary atresia is a disorder of infants in which there is obliteration or discontinuity of the extrahepatic biliary system, resulting in obstruction of bile flow. Untreated, the resulting cholestasis leads to progressive conjugated hyperbilirubinemia, cirrhosis, and hepatic failure. Biliary atresia has an incidence of approximately one in 10,000 live births worldwide. Evidence to date supports a number of pathogenic mechanisms for the development of biliary atresia. An infectious cause, such as by a virus, would seem most pausible in many cases. The clinical observation that biliary atresia is rarely encountered in premature infants would support an agent acting late in gestation. However, no infectious or toxic agent has been conclusively implicated in biliary atresia. Genetic mechanisms likely play important roles, even regarding susceptibility to other specific causes, but no gene whose altered function would result in obstruction or atresia of the biliary tree has been identified. The variety of clinical presentations support the notion that the proposed mechanisms are not mutually exclusive but may play roles individually or in combination in certain patients.