When adults make a joint commitment to act together, they feel an obligation to their partner. In 2 studies, the authors investigated whether young children also understand joint commitments to act together. In the first study, when an adult orchestrated with the child a joint commitment to play a game together and then broke off from their joint activity, 3-year-olds (n = 24) reacted to the break significantly more often (e.g., by trying to re-engage her or waiting for her to restart playing) than when she simply joined the child's individual activity unbidden. Two-year-olds (n = 24) did not differentiate between these 2 situations. In the second study, 3- and 4-year-old children (n = 30 at each age) were enticed away from their activity with an adult. Children acknowledged their leaving (e.g., by looking to the adult or handing her the object they had been playing with) significantly more often when they had made a joint commitment to act together than when they had not. By 3 years of age, children thus recognize both when an adult is committed and when they themselves are committed to a joint activity. (Contains 5 figures, 2 tables and 2 footnotes.)