Decades after his death, Ivan Pavlov (1849-1936) remains one of the best-known scientists of the 20th century. He graduated from the theological seminary in his native Ryazan, Russia, but almost immediately switched to medicine at St. Petersburg University. He became interested in the physiology of circulation and digestion, and from 1902 on studied conditional and unconditional reflexes in animals. He conducted thousands of experiments with dogs, proving that animals, like humans, possess different personalities, tastes, and intelligence. His studies formed the foundation of behavioral psychology and provided the first insights into animal thoughts and emotions. Daniel Todes follows Pavlov's life before and after the Russian Revolution, his stellar scientific career, culminating in the 1904 Nobel Prize for Medicine, and addresses ethical issues related to the physiologist's research on animals.