Observations of 780 third-grade classrooms described classroom activities, child-teacher interactions, and dimensions of the global classroom environment, which were examined in relation to structural aspects of the classroom and child behavior. 1 child per classroom was targeted for observation in relation to classroom quality and teacher and child behavior. These children were enrolled in the ongoing NICHD Study of Early Child Care and Youth Development: 80% of the sample was Caucasian, 22% had a family income-to-needs ratio of 2.0 or less, and 26% of mothers had a high school education or less. Classrooms were observed for a minimum of eight, 30-minute cycles over the course of the day, beginning at the start of the school day, with an intention of observing during academic instruction time. Time samplings of activities, teacher behaviors, and child behaviors as well as global ratings of the classroom environment were obtained. The most frequently observed forms of activity were whole-group instruction or individual seatwork. As expected, the largest portion of time was allocated to literacy-related activities. By a ratio of nearly 11:1, instructional activities (across any content area) were basic-skill-focused versus focused on analysis/inference or synthesis of information. There was wide variation in the frequency of most activities across classrooms. Global ratings also demonstrated significant variability across classrooms. Global and time-sampled codes of teacher behavior and classroom climate were only slightly related to a range of structural factors, such as class size, child-teacher ratio, or teacher experience. Students' engagement in academic activities was higher when classrooms provided more instructional and emotional support. From first to third grade, global aspects of the classroom, such as positive climate or teacher sensitivity, had significant but low stability; time devoted to literacy or math activities was uncorrelated across the two grade levels. These findings suggest that third grade is a highly variable context for children in the United States with a strong emphasis on learning basic skills and that structural factors, such as class size and teacher education and experience, show little relation to the experience of children in classrooms.