Hematopoietic stem cell proliferation and differentiation is regulated by a multistep process involving stem cell self-renewal, commitment, differentiation and maturation. Each process is controlled by a variety of humoral factors and cell-cell interactions, i.e., (i) growth factor and growth inhibitor acting toward the cascade of stem cell proliferation and differentiation, (ii) cytokine network involving cytokine-producing hematopoietic cells and stromal cells, (iii) interaction between hematopoietic cells and bone marrow microenvironment that is consisted of stromal cells and extracellular matrix. Recent advances in cell biology and cell physiology enabled us to understand the roles of these marrow microenvironments on hematopoiesis in molecular basis. We here discuss the complexities of hematopoietic system and the mechanism of the regeneration of hematopoietic tissues in special reference to bone marrow transplantation.