We perceive our own and other people's movements as actions, that is in terms of intentions and goal directedness. When we watch another person who reach for and manipulate an object, it is not the movement itself that is the focus of attention, but rather the object reached for and the goal and sub-goals of the manipulation. This focusing on the goal is of critical importance for our ability to predict actions. It relies on mental models of the rules and regularities that determine our behaviours (theory-of-mind). The purpose of the present project is to learn how infants develop an understanding of their own and other people's actions. Recently research indicates that infants develop interests in other people's action after they themselves have begun to master them. Thus we expect infants to understand external actions like reaching and locomotion before they understand pointing and looking and finally social interaction and communication between other individuals. Another important purpose with the planned studies is to compare the development of normal children's understanding of actions and intentions with children whose development of such skills is impaired. Studies like that does not need to involve infants, but can be carried out on older children with documented social problems, like children with infantile autism.