Hematopoietic stem cells (HSCs) are the immature, pluripotent cells from which all myeloid and lymphoid cell types originate. As stem cells, HSCs are capable of two very different fate choices: self-renewal, ensuring they will persist throughout the lifetime of an organism, and differentiation to mature progeny. Therapeutic applications of HSCs include their routine use in stem cell transplantation to treat hematopoietic malignancies or bone marrow failure. Research and clinical experience have provided tools for the immunophenotypic identification and functional analysis of HSCs and there is increasing evidence suggesting that HSC regulation is greatly influenced by signals from their niches in the bone marrow. Although they represent one of the most rigorously studied stem cell types, still more remains to be known about how HSCs are regulated and respond to stress conditions.