Morphologically, bone marrow is made up of a relatively mature but heterogenous population, fueled by a tiny pool of microscopically unrecognizable stem and progenitor cells. This complex tissue has the responsibility of maintaining our hematopoietic and, to a large extent, immunologic integrity, both of which are indispensable for health and, indeed, survival. Perhaps not surprisingly, bone marrow is the target of genetic, autoimmune, and environmental insults. Although robust, it has only a limited number of responses, one of which is reduction in cellular output, sometimes with superimposed qualitative abnormalities, and this is defined as bone marrow failure. Bone marrow failure is a diverse entity but can be logically explained and classified on a pathophysiologic basis. Thus the major recognizable categories of bone marrow failure are congenital and acquired defects. Each of these is subdivided according to the number of cell lines involved, over and above which the severity of the damage will determine reversibility. In each case, the natural history dictates management, and this ranges from short-term growth factor support to biologic immune response modulation and finally to bone marrow transplantation. In the past, many clinicopathologic variants of bone marrow failure were described, although their etiology was obscure and effective therapy was unavailable. This changed dramatically, however, when experimental hematologists, using radiobiology models, uncovered the dynamic nature of blood formation. Cardinal observations included the way in which spontaneous recovery followed irradiation, the central role played by pluripotential stem cells, and the integral participation of stroma in modulating this entire process. Understanding was refined once bone marrow cultures became available while, in parallel, the use of in-bred mouse strains launched the era of allogeneic transplantation. These approaches were combined, and the broad principles that govern basal or constitutive production emerged. Stem cells, with their characteristic commitment to self-renewal, exist at the apex of a hierarchy and generate a tier of proliferating progenitors that, in turn, give rise to a large postmitotic compartment of precursors that mature into distinctive myeloid and lymphoid lineages. The reserve potential is enormous, and output can be induced to meet even greatly increased demands. These events reflect the interaction of growth factors with a balancing set of negative regulators. The link between such diverse functions resides, to a large extent, in accessory cells and matrix geographically organized in what is now described as the hematopoietic inductive microenvironment. Many details of these meticulously orchestrated processes are obscure.(ABSTRACT TRUNCATED AT 400 WORDS)