Human UDP-glucuronosyltransferases (UDPGTs) are a family of enzymes which detoxify many hundreds of compounds by their conjugation to glucuronic acid, rendering them both harmless and more water soluble, hence, excretable. The level of expression of each UDPGT isoform in the body is the result of interplay between temporal, tissue-specific and environmental regulators. This complexity contributes to the difficulty in predicting the metabolic fate of compounds. Genetic defects and polymorphisms affecting individual isoform activities have deleterious and potentially lethal effects, as exemplified by the severe hyperbilirubinaemia observed in Crigler-Najjar Syndrome. Such severe genetic defects in bilirubin glucuronidation are obvious candidates for antenatal screening and gene therapy.