Today's scientists are showing us how stem cells create and repair the human body. Unlocking these secrets has become the new Holy Grail of biomedical research. But behind that search lies a sharp divide. Stem cells offer the hope of creating or repairing tissues lost to age, disease and injury, yet because of this ability, stem cells hold the potential to incite an international biological arms race. Here the mystery surrounding stem cells is revealed: what they are, how they work and why their use has become so controversial.