Stem cells have the ability to renew and to differentiate into progenitor cells that ultimately form all of the tissues in an organism. The current interest in stem cells, both adult and embryonic, is through the promise that they hold for regenerative medicine. That promise, however, relies on the assumption that stem cells will respond to our modifications of them in ways that we desire. However, experience with interventions in other natural systems, from fishing to antibiotics, shows that acting without thinking about evolutionary consequences is fraught with danger. I will show how to bring the perspective of evolutionary ecology to stem cell biology, using state dependent life history theory and the Hematopoeitic Stem Cell (HSC) system as an example. I will first provides some basics of the HSC system and then provide a simple illustration of how state dependent life history theory (the pro-ovigenic insect) can be developed and connected to experiments. I will then show how elaborations of the theory illuminate why stem cells are so often quiescent and show so much variability in cell cycle times. Finally, I will show how the theory can be applied to predict the penultimate differentiation to myeloid or lymphoid cells of HSC products, and in doing so introduce the stem cell functional response and the fitness control hypothesis. This work reminds us that nothing in biology makes sense except in light of evolution.