Each bone marrow megakaryocyte (MK) releases thousands of platelets into the circulation, and the underlying molecular and cellular mechanisms recently have received intense scrutiny. Genetic studies are beginning to clarify the mechanisms by which transcription factors help distinguish MK progenitors from other blood cell lineages and subsequently confer unique cellular properties. Other investigations demonstrate that platelets are assembled de novo during a terminal phase of MK differentiation in which the cell extends cytoplasmic projections known as proplatelets. This review focuses on the roles of selected transcription factors with key roles in MK differentiation, and on human and murine models of thrombocytopenia that result from impaired MK differentiation. The findings we review help construct a framework to appreciate thrombopoietic mechanisms in the context of underlying lineage and morphologic transitions. Many of these mechanisms are unique to MKs but appear to rely both on genes that are expressed only in that lineage and others that are expressed widely.