This Monograph examines a series of ten studies conducted on the social-cognitive abilities of three young chimpanzees, ages one to four years. In the studies, all previously conducted with human infants, showed the chimpanzees' abilities to understand and imitate the goal-directed actions of others, but not their "rational" choices. Each exhibited the ability to understand the visual perception of others, but not the precise focus of their attention. There was no evidence of any ability to reverse roles with a partner in a collaborative interaction, to comprehend the communicative comprehensions behind a pointing gesture, or to engage in genuine joint attention with others. These results suggest that the ontogeny of human social cognition comprises two relatively distinct trajectories: one for understanding intentional action and perception, common to all apes, and another for sharing psychological states with others in collaborative acts involving joint intentions and attentions, unique to the human species.