Hematopoietic stem cell (HSC) transplantation (HSCT) is used to treat various hematologic disorders. Use of genetically modified mouse models of hematopoietic cell transplantation have been critical in our fundamental understanding of HSC biology and developing approaches for human patients. Pre-clinical studies in animal models provide insight into the journey of transplanted HSCs from infusion to engraftment in bone marrow (BM) niches. Various signaling molecules and growth factors secreted by HSCs and the niche microenvironment play critical roles in homing and engraftment of the transplanted cells. The sustained equilibrium of these chemical and biologic factors ensures that engrafted HSCs generate healthy and durable hematopoiesis. Transplanted healthy HSCs compete with residual host cells to repopulate stem cell niches in the marrow. Stem cell niches, in particular, can be altered by the effects of previous treatments, aging, and paracrine effects of leukemic cells that create inhospitable bone marrow niches that are unfavorable for healthy hematopoiesis. More work to understand how stem cell niches can be restored to favour normal hematopoieis may be key to reducing leukemic relapses following transplant.