This article is about the relationship between teaching and learning. It is based on data from the Project on Learning, in which the experiences of individual students during science and social studies units in 5th and 6th grade classrooms are related to what they learn from these units. Activity theory is used to identify what determines how students participate in classroom learning activities. Typical science and social studies activities have four components: instructions, carrying out the activity, writing a report, and discussing the results. Student participation in classroom activities is a function of how they manage their participation in four interacting systems: the instruction-evaluation system, the peer interaction system, the student's internal cognitive-emotional processing system, and the physical resource system. An example of a typical science activity is used to illustrate how these systems shape student participation. The paper argues that the internalization of the structure and processes of classroom activities shapes the development of learning processes and cognitive abilities. Appended are categories of behavior in group activities and four figures. (Contains 56 references and 8 tables.) (Author/SM)