Because of their need to be stored at room temperature, platelets have a short shelf life and are frequently in short supply. In addition, platelet transfusions are associated with numerous potential risks. Because of these factors, there has been great interest in developing approaches to produce safe and functional ex vivo platelets. The session will start with a review of potential sources of megakaryocytes and platelets and of the biological characteristics of these cells. Next, the use of human embryonic stem cells as a source of platelets will be discussed, as well as the potential for platelets to serve as therapy-delivery vehicles to sites of vascular injury. Finally, practical current approaches to optimize ex vivo platelet production will be reviewed.