In this study, we addressed the question of the nature of the information needed by 13-month-old infants to understand another agent's intentions. In two experiments, an experimenter was either unable or unwilling to give a toy to an infant. Importantly, an implement (a gutter in which the toy could roll down toward the infant) was used to make the experimenter's behavior as similar as possible in the two conditions. When the experimenter remained still in both conditions, infants did not behave differently according to the experimenter's intentions, suggesting that they did not infer them. By contrast, in a second experiment, where the experimenter performed an action directed toward the gutter in both conditions, the infants looked away from the experimental setting more often and longer in the "unwilling" condition than in the "unable" condition. They also looked more toward the experimenter in the unable condition than in the unwilling condition. Therefore, we conclude that an agent's intentional attitude can already be inferred by a 13-month-old provided that this intention is concretely shown through a goal-directed action. (Contains 2 figures.)