Understanding the processes that regulate the developmental program of normal stem cells and those that initiate proliferative diseases such as leukemia remains one of the major challenges in biology. Progress to address these major questions in the human hematopoietic system have been hampered, until recently, by the lack of in-vivo assays for normal and leukemic stem cells. The recent development of methods to transplant normal and leukemic human hematopoietic cells into immune-deficient mice provides an important approach to identify, characterize and purify stem cells. This review will focus on the development of assays for normal and leukemic human stem cells and on the new insights these assays are beginning to provide on the organization of the human stem cell hierarchy and mechanisms of leukemogenesis.