The successful engraftment of human hematopoietic cells into immune-deficient mice offers a novel approach to characterize the developmental program of human hematopoiesis. While it is not yet possible to achieve high-level engraftment of all human lineages, several methods have been developed to successfully engraft human lymphoid cells and reconstitute partial immune function. In addition to mature cell types, there is evidence that progenitors and perhaps stem cells can engraft the murine bone marrow. Recent work suggests that provision of exogenous human cytokines significantly increases the level of human cell engraftment and stimulates the development of multiple lineages. Progress has also been made to establish animal models of human hematopoietic diseases such as leukemia, autoimmunity, and infectious diseases.