This chapter considers the transport of unconjugated bilirubin-IXα, in the mammalian organism, from the site of entry of the pigment into the blood stream to its site of en-zymic conversion. Central to the discussion are the associations of the pigment with binding proteins and biomembranes. Transport aspects of formation and conjugation of bilirubin, and of excretion of bilirubin conjugates are not considered because of lack of appropriate experimental data. Studies of bilirubin transport are important in the understanding of how mammalian organisms handle the toxic waste product and may help to explain its hepatophilic character. In mammals, the transport pathway of bilirubin ends at enzymic sites where the chemical identity of the pigment is changed, mainly through the action of hepatic bilirubin uridine-5’-diphosphate (UDP)-glucuronyltransferase which catalyzes the formation of mono- and diglu-curonides. Binding of bilirubin to cytomembranes may be of particular significance in bilirubin encephalopathy.