In this paper, I analyze the theoretical and empirical foundations for how class composition affects gifted students' academic outcomes. First, I define the term composition effect and distinguish it from other context effects. Based on this definition, I present the mechanisms behind composition effects while drawing distinctions between resource-related, peer-related and teacher-related composition effects. Subsequently, I discuss theories and empirical results of international school effectiveness studies in terms of their relevance to increasing equity in gifted education. Ability groups, especially gifted classes, have an advantage for the promotion of gifted students. However, due to selection mechanisms, low-income students drop out of these support programs very early in their school careers. Behind the supposed negative influence of ethnic composition on school performance, a socioeconomic effect can be assumed. This is thus the most important composition effect in regard to inequality in talent development. However, there are ongoing efforts to ensure greater equity specifically in gifted education.