The nineteenth century science of teratology concerned itself with the study of malformations or "monstrosities", as they were then called. The first major contribution to the field was the work of Isidore Geoffroy Saint-Hilaire, Histoire Generale et Particulie`re des Anomalies de l’Organisation chez l’Homme et les Animaux, published in 1832, whose classifications formed the basis for the later experimental science of teratogeny, the art of reproducing monstrosities in animal embryos. In this article, I will argue that recent developments in the field of regenerative medicine can be situated in the tradition of teratological and teratogenic studies dating back to the nineteenth century. In particular, I will be interested in the historical link between studies in teratogenesis (the artificial production of teratomas) and stem cell research. Recent advances in stem cell research, I will suggest, return us to the questions that animated nineteenth century investigations into the nature of the monstrous or the anomalous. In the process, our most intuitive conceptions of "life itself" are undergoing a profound transformation.