The maintenance of the various blood cell populations in mammals is achieved by the proliferation and differentiation of precursor cells located primarily in the bone marrow. These precursor cells are all derived from a common haemopoietic stem cell population that is established during embryogenesis and functions for the lifetime of the organism. An overview is given of the various assay systems for haemopoietic stem cells, how these assays have contributed to understanding the considerable heterogeneity within the stem cell compartment, the regulation of stem cells and the development of the haemopoietic system.