The main concern is a longstanding one in classroom instruction--the determinants of effective team performance. The paper explicitly examines the effect of teacher-controlled factors on the use and functioning of student teams. From a sample of 500 undergraduate students, data are obtained on aptitude, diversity, instability, motivation, personality style, size, and performance. The regression results suggest that team motivation and instability, which are both partly controlled by the instructor, are particularly important in determining a team's performance. An implication is that instructor decisions about team make-up and incentives can have a significant impact on student achievement.