The term "thrombopoietin" (TPO) was first coined in 1958 but, despite several cytokines having thrombopoietic effects, the major megakaryocytic growth and development factor was not cloned until 1994. Wendling and colleagues (Virol, 1986; 149: 242-246) recognised a novel murine viral oncogene, v-mpl, responsible for a myeloproliferative syndrome. The corresponding human proto-oncogene, c-mpl, codes for a member of the haemopoietic cytokine receptor family. Its ligand is TPO. The structure and function of TPO, together with its action in stimulating proliferation and maturation of megakaryocytes, are discussed in detail in this review which also details the pre-clinical usage and clinical potential of TPO as a platelet sparing agent in haemato-oncological practice.