Our tissues constantly lose cells because of normal wear-and-tear but also because of accidents and diseases. Specialized stem cells reside in every tissue to replace lost cells by new, healthy cells. Over time, stem cells gradually lose the capacity to do so, the reason why our tissues eventually undergo aging. When -on the other hand- stem cells become hyperactive, this leads to the formation of benign and malignant tumors. I will address basic aspects of stem cell biology and the roles of stem stem cells in health and disease. I will also demonstrate how it has become possible to ‘tame’ stem cells in the lab, to grow them in a dish, to genetically modify them and to give them back to diseased individuals.