Although the cellular physiology of megakaryocyte and platelet production has been understood for nearly a decade, the humoral basis of this process has remained enigmatic until recently. “Thrombopoietin — At Last” heralded one editorial¹ when the cloning of the chief regulator of megakaryocyte and platelet production was reported.²–⁴ The availability of the purified hormone has made possible new insights into many aspects of blood-cell development and provided a way to increase platelet production in patients with bone marrow failure. This review focuses on the physiology of thrombopoietin and megakaryocyte production, with a view to understanding how this information can . . .