Eugenics—the theory that we can improve future generations of humans through selective breeding—was one of the most controversial movements of the early 20th century. Often tied to racist beliefs and nativist desires to limit immigration, the eugenics movement attracted some of the most prominent scientists and social reformers of the day. This encyclopedia brings into one place concise descriptions of the leading figures, organizations, events, legislation, publications, concepts, and terms of this vitally important period historical movement. Although focused on the United States during the heyday of the movement, the encyclopedia includes material on international event as well as connections to important contemporary issues such as genetic engineering, family balancing, and the possibility of human cloning.