The experiment reported here investigated infants' concept of intention, as well as the relation among intention understanding, general productive vocabulary, and internal state language production during the 2nd year. Results from an imitation task indicated that 18-month-olds are better able to differentiate between intentional and accidental actions than 14-month-olds. Although there was no relation between infants' performance on the intention task and their general productive vocabulary, internal state language production at 30 months was predicted by infants' ability to differentiate between intentional and accidental actions about a year earlier. These findings shed light on the developmental progression of infants' concept of intention, as well as on the continuity between infants' understanding of intentional action and their ability to use internal state words. (Contains 2 figures.)