This study explores the importance of the group context in the emergence of leadership, dominance, and group effectiveness in children's collaborative learning groups. Ten 3-person work groups performed a collaborative math activity. Using achievement goal orientation (Ames, 1992; Maehr and Midgley, 1996; Pintrich and Schunk, 1996) as a framework, six groups performed the math task under a mastery condition, which emphasized learning and improving. Four groups performed under a performance condition, which emphasized competition and social comparison. The groups were videotaped and fully transcribed. The group interactions were analyzed qualitatively (Glaser and Strauss, 1967; Strauss and Corbin, 1990). The emergence of leadership and dominance varied under mastery or performance group conditions. More specifically, under the performance condition, group members exhibited more dominance and negative behaviors, such as arguing, off-task behavior, and group member isolation. Under a mastery condition, group members exhibited more leadership and positive behaviors, such as giving and seeking help, talking about math strategies, and staying focused on the task. Group effectiveness also varied under mastery and performance group conditions. Under the performance condition, groups were not as effective in completing the math task because of member dissonance, isolation, lack of communication, and dominance. Under the mastery condition, groups were more effective, with more communication among all members and a shared responsibility in completing the math task. Implications for classroom practice are discussed. (Contains 59 references.) (Author)