What makes us human? This question has occupied people for millennia. A con-clusive answer continues to elude us as we learn more about ourselves and other animals. A series of important discoveries over the last 50 years have led us to largely abandon the search for single traits that are unique to humans. We now know that tool use, language-like communication, lethal intergroup aggression, and an ability to anticipate future events can also be found in other species. How-ever, humans are still quite different from other animals. So, the principal question has become: ‘What is the nature and the width of the gap that separates humans from primates and other animals?’ This edited volume features a collection of es-says by primatologists, anthropologists, biologists, and psychologists who offer some partial answers to this question. In this introductory chapter, we briefly out-line the background of this fundamental question about human universals and ex-plain our emphasis on behavioral traits.