Stem cells build our bodies during development, starting from a fertilized oocyte. Subsequently, they maintain and repair our organs for a lifetime. Thus, stem cells hold the promise of eternal life. While this may still be in the distant future, stem cells are rapidly changing the face of biomedical science and are finding applications in the clinic. In this article, I discuss the principles of stem cell biology and the mini-organs that can be grown from stem cells in a petri dish.