How did the notions of “race” and “ethnic group,” under the cover of scientific legitimacy, get used for political ends? This work retraces the history of biological conceptions of society and their racist and eugenicist applications from the end of the nineteenth century to the post-Second World War epoch. Andre Pichot analyzes the relationship between science, politics and ideology, through the examination of specific cases: from Nazism to the various eugenicist research programs launched or financed by eminent scientific organizations from the beginning of the twentieth century onwards. And, today, with the mapping of the human genome and rapid advances in gene therapies, he warns that the dream of a “pure society” is in danger of resurrection.