Thrombopoietin (TPO) is a growth factor produced by the liver that is critical for every stage of platelet differentiation and production in the bone marrow. Early forms of recombinant TPO were withdrawn before licensure because of adverse events, but a number of safer new drugs with TPO-like function are emerging for use in chronic Idiopathic Thrombocytopenic Purpura (ITP). This audioconference will begin by reviewing the physiology of platelet production, the role of TPO, and the mechanisms of action of the newer TPO agonists. While they may not be useful in acute thrombocytopenia or marrow failure, TPO agonists might have a role in the treatment of chronic thrombocytopenic conditions such as myelodysplasia and liver disease. Whether these drugs have potential to decrease the need for platelet transfusions, and the possibility of use in platelet donation, will also be reviewed.