In the past 40 years our concepts about hemopoiesis have been changed dramatically. The results of bone marrow transplantation into lethally irradiated mice since the mid-fifties suggested the existence of a hemopoietic stem cell, which was initially identified as a spleen colony forming cell (CFU-S). Later experiments showed that the stem cell compartment is rather heterogeneous and that the most primitive stem cell, unlike the CFU-S, has the ability for long-term engraftment of an irradiated recipient. Daughter cells of such primitive quiescent stem cells lose their capacity for self-generation gradually with each mitosis and become more and more committed to a specific differentiation lineage. In vitro culture techniques in a serum-free semi-solid medium enabled the establishment and analysis of specific hemopoietic growth factors. Such factors, which are essential for the maintenance, proliferation and differentiation of progenitor cells and the functional activity of mature cells can now be produced with recombinant DNA techniques in pure form and large quantities. Hemopoiesis requires an appropriate microenvironment, consisting of various stromal cell types and an extracellular matrix. Intercellular contacts, adhesion of cells and growth factors to the matrix molecules seem essential in the regulating action of this hemopoietic microenvironment. In long-term bone marrow cultures the development of a stromal hemopoietic microenvironment can facilitate long-term maintenance of stem cells and hemopoietic differentiation. For bone marrow transplantation and infusion of hemopoietic growth factors many clinical indications are well established and our possibilities to interfere in the regulation of hemopoiesis are still growing.