The availability of thrombopoietin (TPO) in recombinant form has revolutionised the study of megakaryocytopoiesis and provided an exciting new reagent for clinical evaluation. Through the application of gene targeting technology, the production of mice lacking TPO or its receptor c-Mpl has provided valuable insights into the physiological roles of TPO signalling. The near identical phenotype of c-mpl^−/− and TPO^−/− mice provides strong biological evidence that TPO is the sole c-Mpl ligand and uses no other additional receptor itself. TPO^−/− and mpl^−/− mice are severely thrombocytopenic indicating that TPO is the primary physiological regulator of platelet production in vivo. The physiological basis for this platelet deficiency has been further defined by analysis of megakaryocytes and committed progenitor cells, the numbers of which are also reduced in these mutants. The platelets that are produced in the absence of TPO signalling are morphologically and functionally normal and residual production is sufficient to prevent bleeding and allow a normal lifespan. Thus, TPO^−/− and mpl^−/− mice also reveal that important TPO-independent mechanisms exist that control platelet production in vivo, and these mice are ideal models to explore the nature of these alternative regulators. The mechanisms regulating the circulating levels of TPO have also been elucidated in these mice, highlighting the central role of c-Mpl mediated internalisation and degradation. The unexpected observation that progenitor cells of all hemopoietic lineages are produced in reduced numbers in TPO^−/− and mpl^−/− mice has also led to studies that uncovered a central role for TPO signalling in hemopoietic stem cell regulation.