Alagille syndrome (ALGS, OMIM 118450) is a multisystem disorder due to defects in components of the Notch signalling pathway. Its main clinical and pathological features are chronic cholestasis due to paucity of intrahepatic bile ducts, peripheral pulmonary artery stenosis, vertebral segmentation anomalies, characteristic facies, posterior embryotoxon and dysplastic kidneys. It is transmitted in an autosomal dominant pattern of inheritance with variable expressivity that accounts for its phenotypic variability. Diagnosis may be easy if all the clinical signs are present, while it can be difficult in some patients who do not show unequivocal classic features of the disorder. The short-term prognosis is conditioned by the severity of the possible complex cardiopathy and by the presence of an early and severe cholestasis, while the prognosis in the long term remains uncertain because of the risk of developing a terminal renal insufficiency and cerebral vascular complication.