Traditional empirical studies on developmental processes in school tend to view contributions of teachers, peers, and the classrooms' social structure (and even parent effects) as if all were independent of one another. As this Special Issue demonstrates, however, these processes are more complex. When classroom interactions are seen as the "engine" of development (Bronfenbrenner & Morris, 1998), attention to the invisible hand of the teacher is necessary. The contributions to the issue show empirical strategies common to such a perspective (use of cross-informant data, attention to interconnections between different kinds of relationships and to interconnections between the social and academic domains), and they all highlight common conceptual features (focus on relationships and social structure, on reciprocal processes, and on the person characteristics of teachers and peers). Taken together, the findings from the studies should have a collective impact on teacher education.