The events underlying the commitment and differentiation of megakaryocytes are poorly understood, particularly with respect to understanding the biochemical and molecular mechanisms regulating this process. These regulatory events begin with the interaction of multiple microenvironmental signals (eg, cytokines, extracellular matrix) with specific cell surface receptors, extend through a signal transduction cascade, and end with transcriptional activation of megakaryocyte-specific genes. This article focuses on the cellular, biochemical, and molecular control of megakaryocyte differentiation events, whereas data on the ligands, receptors, and signal transduction are found elsewhere in this issue. The first area discussed is the classification of functional categories of the cells of the megakaryocyte lineage: identifying those cells which respond to proliferative signals, those which mark the transition from the proliferating cell compartment to mature cells, and the mature, post-mitotic platelet-shedding cells. The transitional cells, the pro-megakaryoblasts, are covered in some detail as these cells are physiologically important both in their early response to thrombopoietic stress, and for their unique capacity to continue to synthesize DNA during their differentiation. Finally, recent data on the control of the process of megakaryocyte polyploidization, as well as the molecular control of megakaryocyte commitment are discussed.