Bleeding syndromes that arise through an inherited defect of platelet production constitute a heterogeneous group of rare platelet disorders of growing importance. 1, 2 Some, including the Bernard-Soulier syndrome (BSS) and Wiskott-Aldrich syndrome (WAS), associate a low circulating platelet count with a deficiency in a known functional protein (Table 1). In others, platelet dysfunction has not been shown and the genetic cause lies in the inability of megakaryocytes (MK) to mature and to produce platelets in sufficient numbers. In congenital amegakaryocytic thrombocytopenia, there is an increased tendency towards the development of leukemia, while in others such as the Jacobsen syndrome, the defects extend outside megakaryocytopoiesis and interfere with the development and/or functioning of major organs. In many of these rare diseases, the low platelet count is accompanied by changes in platelet morphology including the presence of enlarged or giant platelets. The elucidation of the genetic basis of familial thrombocytopenias is providing basic knowledge of how MK develop from the pluripotent hematopoietic stem cell (HSC) under the influence of thrombopoietin (TPO) and other cytokines. This short review will mainly deal with the biology and genetics of inherited thrombocytopenias.