Supervision is recognized as a critical component of preservice training in health service psychology (HSP), particularly during the internship experience. However, little is known empirically about what constitutes effective internship supervision for school psychologists. For this constructivist grounded theory study, we interviewed 16 highly accomplished internship supervisors regarding their approaches to supervision. Data from supervisors' former interns and colleagues were also analyzed. Findings indicated that supervisors' personal and professional selves are central to how they implement supervision within a broader developmental and ecological context. Fundamental aspects of supervision emerged as bidirectional, including establishing and maintaining a strong working alliance, planning the internship experience, and processes of learning for both interns and supervisors. The grounded theory is discussed with reference to the HSP supervision literature, and specific implications are drawn for the practice of and further research on supervision in school psychology.