The biologist Jacques Loeb (1859-1924) helped to shape the practice of modern biological research through his radical emphasis on reductionist experimentation. This biography traces his career and convincingly argues that Loeb''s desire to control organisms, manifested in studies of both reproduction and animal behavior, contributed to a new self-image for biologists. The author places Loeb''s experiments and the controversies they generated in their intellectual and institutional contexts, tracing his influence on the development of behaviorism, genetics, and reproductive biology.