Our long-term goal is to understand the functional heterogeneity of adult stem cells and to redefine how tissues are maintained, repaired, and regenerated. Using hematopoietic and skeletal stem cells as model systems, we investigate how distinct stem cell subpopulations contribute differently to steady-state homeostasis, transplantation, and injury repair. Our work has shown that stem cells with similar phenotypes can play markedly different roles under physiological and stress conditions, and has identified conserved fate transitions that underlie effective tissue repair. These findings challenge classical paradigms of stem cell biology and provide a conceptual foundation for precision regenerative medicine. Moving forward, we aim to identify the true cellular sources that drive tissue repair and regeneration, and to translate this knowledge into controllable therapeutic strategies for regenerative medicine.