Although thrombopoietin (Tpo) was already described in the early sixties, it took more than 30 years before it was cloned. It became possible after the recognition and cloning of its receptor, Mpl. In the past few years new information about Tpo and Mpl has accumulated rapidly. Structure, biosynthesis and tissue expression have been elucidated. The central role of Tpo in the regulation of megakaryocytopoiesis and platelet production has become clear, as has the way in which this is accomplished. Tpo appears to be an important growth factor for haematopoietic stem cells as well. Thus, it may become one of the most important factors in stem cell transplantation. Finally, the signal transduction mechanisms involved and the way in which Tpo affects platelets and their precursors functionally have been studied in detail. The cloning and characterization of thrombopoietin and its receptor Mpl is one of the most important advances in the haematology of the nineties.