Since the first descriptions of Alagille syndrome (syndromic bile duct paucity) 30 years ago, our appreciation of the clinical variability and complexity of this disorder has grown. In addition to the liver, Alagille syndrome is associated with abnormalities that involve the heart, eye, skeleton, kidneys, and the increasing importance of abnormalities of the central nervous system is being recognized. The developmental nature of the disorder has been proven with the identification of the disease-causing gene, Jagged1. Jagged1 is a cell surface protein that functions in an embryologically important signaling pathway, known as the Notch signaling pathway. Identification of the role of Jagged1 (JAG1) in the etiology of Alagille syndrome has improved diagnosis for this variably expressed disorder. In this review, we summarize information on the range of clinical abnormalities of the liver and other affected organs in affected individuals. Genetic studies have demonstrated the range of defects in JAG1 that cause Alagille syndrome. Mutations in JAG1 can be identified in 70% of Alagille syndrome patients, and they are inherited in 30-50%. These mutations include total gene deletions as well as mutations (frameshift, missense, and nonsense) in almost all regions of the 26 exons of the Jagged1 gene. This review focuses on clinical and genetic features of Alagille syndrome.