This paper describes SOFIT (System for Observing Fitness Instruction Time), an observation instrument designed to assess variables associated with students’ activity levels and opportunities to become physically fit in physical education. SOFIT involves the direct observation of classes while simultaneously recording student physical activity levels, curriculum context variables, and teacher behavior. The paper reports the reliability, validity, and feasibility of using the instrument, as well as data from using SOFIT to assess 88 third- and fourth-grade classes.