Human twins have many meanings and different histories. They have been seen as gods and monsters, signs of danger, death, and sexual deviance. They are taken as objects of wonder and violent repression, the subjects of scientific experiment. Now millions are born through fertility technologies. Their history is often buried in philosophies and medical theories, religious and scientific practices, and countless stories of devotion and tragedy. In this history of superstitions and marvels, fantasies and experiments, William Viney—himself a twin—shows how the use and abuse of twins has helped to shape the world in which we live. This book has been written not just for twins, but for anyone interested in their historical, global, and political impact.