The fear of degeneration, which was the basis of propaganda for eugenic programs and racial hygiene policies, spread throughout the Western world beginning in the mid-nineteenth century and peaked in the early decades of the twentieth century. It was a fear fomented by physicians, psychiatrists, biologists, and naturalists. In this article, I analyze the most significant stages that marked the birth of degeneracy theory in France and the political use that was made of it, first in the United States and then in Europe.