Classrooms can be described in terms of the recurrent activity structures teachers use. Activity structures such as reading circle or seatwork have different functions and possess rules and norms to guide the behavior of individuals in the structure. A taxonomy and description of eleven activity structures was derived from observations of over 1200 activity structures in elementary classrooms. Each structure was coded for duration, number of students, the teachers role, student attention, etc. The activity structures perspective allows for a more precise description of classrooms than has previously been possible. This is important for ATI research, which has suffered from a lack of precision in specifying environments or treatments. The usefulness of this perspective for ATI research is described. Hypotheses about aptitudes that may foster or hinder achievement in different activity structures are suggested.Studying life in classroorns is like studying life in the United States of America. To know that a person lives in the United States is to know a great deal. One can quickly bring to bear knowledge about norms for the behavior of Americans, for example, their tolerance levels for political protest, their religious preferences and the strength of their religious convictions, their attitude toward divorce, their preferences in popular music, their treatment of minorities, etc. In general, we have some understanding of the context in which that person lives because of our experience with Americans. The same level of understanding occurs when we study students in Mrs. Maxwell's third grade class. If we have had a chance to experience life in Mrs. Maxwell's classroom, then we probably know the norms for behavior in her class. We would know something about her use of reinforcement and her perceived warmth, her methods of accountability for students, and the academic press in the classroom. We would probably also know something about how fairness is perceived in that classroom and whether work was conducted in a businesslike manner.Within America, however, there are places as different as San Francisco and Iowa City, Las Vegas and Fargo, Honolulu and Houston. In Mrs. Maxwell's class there are settings as different as the reading circle and the listening center, discussion group and seatwork, playground and computer drill, movie viewing and housekeeping. Each city in America and each classroom setting in Mrs. Maxwell's class has associated with it its own norms for behavior. At the global level we understand something about life in the United States and about life in Mrs. Maxwell's classroom. But, it is not until we have information about the sub-or mini-environment: the city, neighborhood or classroom setting, that we begin to feel comfortable about understanding or predicting the behavior of people. This article is directed toward increasing our understanding of the more restricted subenvironments of classrooms. The article begins with a discussion of the importance of activity structures, the name for these subenv...