Gilbert syndrome (GS) is a very common pathology, characterized by the presence of non-conjugated hiperbilirubinemia, in the absence of hepatic dysfunction or haemolysis. Its diagnosis, initially of presumption, was changed when in 1995 the first mutation in UDP-glucuronosyltransferase 1 gene was described. Particularly, the (TA) duplication in the promoter region of the gene, which was described as the principal cause of GS in all studied Caucasian populations. In this paper a revision of some aspects of bilirubin metabolism will be done, as well as, the most important hereditary pathologies associated with their metabolism, with particular emphasis to GS.