Summary: Reference group effects on self-concept have been examined in a multitude of empirical studies in various educational settings. Effects of this kind have been most rigorously demonstrated by Herbert W. Marsh, who introduced the term big-fish-little-pond effect (BFLPE) to describe the finding that students' self-concept is negatively affected by the achievement level of other students in the same class or school. The phenomenon of the BFLPE is supported by a large body of empirical research. In this contribution, we first discuss mechanisms underlying the BFLPE. Second, we address reference group effects on educational outcomes other than self-concept. Finally, we briefly discuss educational implications of the BFLPE.