Action is central to human experience. While considerable research suggests that action experience shapes cognition, we know relatively little about how this link emerges—particularly early in development when the action system undergoes dramatic change. In this dissertation, I integrate neural and behavioral measures to investigate how the action system is linked to social cognition in the first year of life. In chapter 1, I argue that in order to better understand the role that the action system plays in infant social cognition, we need clear evidence that the variability in the mu-ERD response is about infants’ own motor development. To do so, I investigate the relation between the mu-ERD response and several aspects of motor development. I find a specific link between infants’ ability to plan their actions and the sensorimotor mu-ERD response. In chapter 2, I investigate whether there is a functional link between the mu-ERD response and action encoding. I demonstrate that sensorimotor mu-ERD response is selectively present when infants encode others’ goals. In chapter 3, I ask: how developmentally generative is the connection between the motor system and social cognitive development? Here I examine the longitudinal relation between the sensorimotor mu-ERD response in infancy and preschool theory of mind. This work suggests that the action system shows some links to children’s explicit understanding of perception, but overall there are not strong links to abstract mental state reasoning. Taken together, these findings provide novel insight into the link between the infant mu-ERD response, action, and perception.