Hematopoiesis is a dynamic process, which generate in the range of 10(9) cells/kg each day of erythroid and myeloid cells respectively. In vitro assays that were developed 20 years ago, have been used to define factors that can stimulate growth and differentiation of bone marrow (BM) derived progenitor cells. These growth factors for hematopoiesis were termed Colony Stimulating Factors (CSFs) since the assay system was to induce colonies. With the application of molecular biologic approaches, the genes encoding for these CSFs have been localized and cloned. Production of CSFs and other soluble signal substances (cytokines) as pure proteins have led to important insights into how hematopoiesis is regulated by a complex network made up by interactions between cells and cytokines. The availability of CSFs in clinically useful amounts has also led to clinical trials with new strategies for treating hematopoietic dysfunctions, congenital or acquired. Because others have recently reviewed clinical applications or basic science studies on the colony stimulating factors, we will summarize the two with focus on common features between the different CSFs.