The analysis of a database containing videotaped student interactions during science lessons, provides evidence for three claims about the relationship between gestures and talk. First, when a scientifically appropriate discourse has not yet been developed, students' gestures describe and explain scientific phenomena. Second, when a scientifically appropriate discourse starts to emerge, associated utterances are preceded by iconic and deictic gestures. Third, as students become more familiar with a scientific domain, their scientific discourse takes on a greater role while their gestures begin to coincide with their talk.