Stem cell biology has many roots, and the current interest in the possible medical and pharmaceutical applications of pluripotent stem cells has far removed origins in the biology of a rare but peculiar type of tumor, the teratomas. The identification of their stem cells and their relationship to the early embryo paved the way, first in the mouse and later in humans, to the development of embryonic stem (ES) and induced pluripotent stem (iPS) cells, and to approaches for controlling their differentiations. More recently, the recognition of genetic change and culture adaptation of these cells after prolonged culture has returned us to those cancer roots.