Erythropoiesis is the process leading to the formation of red cells. Our understanding of erythropoiesis regulation has been due to the characterization of the erythroblast progenitor cells BFU-E and CFU-E. These cells cannot be recognized morphologically, but they proliferate and differentiate in vitro under the effect of growth factors. They correspond to different steps in erythroblast differentiation which is in fact subjected to different regulations. BFU-E cells are the earliest cells of erythropoiesis and are regulated by such growth factors as GM-CSF, Il-3 or Il-9, which are not specific to erythropoiesis. As they differentiate, the BFU-E cells become sensitive to erythropoietin, the erythropoiesis-specific hormone secreted mainly by the kidney in response to hypoxia. The action of erythropoietin and that of other growth factors are synergistically reinforced by a non erythropoiesis-specific growth factor called Steel factor which plays a very important physiological role since defects in its synthesis caused by genetic anomalies produce a lethal anaemia in mice. CFU-E cells, which precede proerythroblasts, are the most sensitive to the action of erythropoietin both in vitro and in vivo. A better knowledge of erythropoiesis regulation mechanisms should help us in the understanding of the physiopathology of some congenital or acquired diseases, such as erythroblastopenia or polycythaemia vera.