Hematopoietic stem cell transplantation has traditionally been used to reconstitute blood cell lineages that had formed abnormally because of genetic mutations, or that had been, eradicated to treat a disease such as leukemia. However, in recent years, much attention has been paid to the new concept of "stem cell plasticity," and the hope that stem cells could be used to repair damaged tissues generated immense excitement. The field is now in a more realistic and critical period of intense investigation and the concept of cell fusion to explain some of the observed effects has been shown after specific types of damage in liver and muscle, both organs that contain a high number of multinucleate cells. The field is still an extremely exciting one, and many questions remain to be answered before stem. cell therapy for tissue repair can be used effectively in the clinic. Immune deficient mouse models of tissue damage provide a system in which human stem cell migration to sites of damage and subsequent, contribution to repair can be carefully evaluated. This chapter gives detailed instructions for methods to study human stem cell contribution to damaged liver and to promote repair of damaged vasculature in immune deficient mouse models.