Within the human body millions of mature blood cells are produced every second to replace blood cells lost due to their short lifespan. This process is critical in order maintain normal blood cell levels required to distribute oxygen to peripheral tissues, prevent bleeding and protect against, or fight, infectious agents. In adults, the production of mature blood cells, haematopoiesis, takes place in the bone marrow and is driven by rare hematopoietic stem cells. Haematopoietic stem cells are functionally defined through their unique ability to self-renew and give rise to mature blood cells through differentiation which generates series of intermediate progenitor cells increasingly restricted towards one blood cell type. This chapter focuses on the role of haematopoietic stem cells in human haematopoiesis and clinical medicine. An outline of experimental assays allowing identification and characterization of human haematopoietic stem cells and factors regulating their ability to produce mature blood cell lineages is described.