Megakaryocytic progenitor cells only complete a limited number of mitotic events; early in megakaryopoiesis, these cells loose their capacity for cell division and acquire the ability for endoreduplication--a phenomenon that is unique to the megakaryocytic lineage. There is growing evidence, that at least two humoral activities affect proliferation of progenitor cells and maturation of its progeny. However, the cytokines that mediate these functionally defined, overlapping activities are not yet precisely known. This review summarizes current understanding of these processes and will discuss the interactions of megakaryocytic cells with the bone marrow microenvironment as well as their role in disorders of hematopoiesis.