Proper platelet function is essential for hemostasis. However, understanding platelet function is complicated by the fact that platelets are anucleate and therefore not amenable to direct genetic manipulations. To study platelet function, several laboratories have developed CHO cell lines expressing platelet proteins or used megakaryocyte-like cell lines. However, these cell culture models are unable to mimic critical platelet functions, most notably agonist-induced activation of integrin alphaIIbbeta3. Mature megakaryocytes, which are platelet precursors, express platelet-specific proteins, and the function of such proteins and signaling pathways appears conserved between the two cell types. Murine megakaryocytes have been successfully differentiated in cultures from bone marrow, fetal liver, and embryonic stem (ES) cells, while human megakaryocytes have been cultured from human cord blood, peripheral blood, and ES cells. The various sources of megakaryocyte progenitors provide choices to researchers, allowing them to access the most applicable systems. As examples, both bone marrow-derived and ES cell-derived murine megakaryocytes have been used to study proteins involved in integrin alphaIIbbeta3 regulation such as CIB1 and H-Ras. Therefore, megakaryocytes have provided an invaluable resource for better understanding the biology of platelets. In this chapter, we will describe: (1) approaches to obtain, generate, and characterize megakaryocytes, (2) molecular manipulation of these cells to elevate or decrease expression levels of specific proteins, and (3) current uses and future applications of megakaryocytes.