Speakers monitor their own speech and, when they discover problems, make repairs. In the proposal examined here, speakers also monitor addressees for understanding and, when necessary, alter their utterances in progress. Addressees cooperate by displaying and signaling their understanding in progress. Pairs of participants were videotaped as a director instructed a builder in assembling 10 Lego models. In one group, directors could see the builders’ workspace; in a second, they could not; in a third, they gave instructions by audiotape. Two partners were much slower when directors could not see the builders’ workspace, and they made many more errors when the instructions were audiotaped. When their workspace was visible, builders communicated with directors by exhibiting, poising, pointing at, placing, and orienting blocks, and by eye gaze, head nods, and head shakes, all timed with precision. Directors often responded by altering their utterances midcourse, also timed with precision.