Human stem cell models (CD34+ hematopoietic progenitors, embryonic stem cells and induced pluripotent stem cells (iPSCs)) are powerful tools for the study of megakaryopoiesis and thrombopoiesis, particularly in situations where mouse models are unavailable or do not accurately recapitulate human physiology or development. In the first part of this thesis, we identified and characterized novel megakaryocyte (MK) maturation stages in MK cultures derived from human stem cells. An immature, low granular (LG) MK pool (defined by side scatter on flow cytometry) gives rise to a mature high granular (HG) pool, which then becomes damaged by apoptosis and GPIbα (CD42b) shedding. We define an undamaged HG/CD42b+ MK subpopulation, which endocytoses fluorescently-labeled coagulation factor V (FV) from the medium into alpha-granules and releases functional FV+CD42b+ platelet-like particles in vitro and when infused into immunodeficient mice. Importantly, these FV+ platelets have the same size distribution as infused human donor platelets and are preferentially incorporated into clots after laser injury. Using drugs to protect HG MKs from apoptosis and CD42b shedding, we also demonstrate that apoptosis precedes CD42b shedding and that apoptosis inhibition enriches the FV+ HG/CD42b+ MKs, leading to increased platelet yield in vivo, but not in vitro. These studies identify a transition between distinct MK populations in vitro, including one that is primed for platelet release. Technologies to optimize and select these platelet-ready MKs may be important to efficiently generate functional platelets from in vitro-grown MKs. In the second part of this thesis, we used patient-specific iPSCs to model Thrombocytopenia Absent Radius (TAR) syndrome, a rare congenital disorder characterized by low platelet counts and bilateral absence of the radius. We generated several iPSC lines from patients and controls and confirmed that the patient lines had decreased expression of RBM8A, the candidate disease gene for TAR syndrome. We differentiated patient and control iPSCs to hematopoietic progenitor cells (HPCs) and MKs and showed that HPCs derived from TAR iPSCs exhibited decreased MK colony-forming potential but differentiated normally into MKs in liquid culture. When we restored RBM8A expression in TAR iPSCs using a doxycycline inducible system, we saw no effect on megakaryopoiesis. We then knocked out RBM8A in TAR iPSCs and found that a complete deficiency in RBM8A expression was lethal for iPSCs, HPCs, MKs and erythrocytes. These studies suggest that we were unable to determine a specific role for RBM8A in primitive (embryonic) megakaryopoiesis and perhaps future studies to direct iPSCs towards the definitive (adult) MK lineage may better elucidate RBM8A’s role in TAR syndrome.