This review of research was prompted by the widespread belief that at least in a significant number of secondary schools, department heads are an underutilized, if not untapped, source of instructional leadership, the type of leadership critical to secondary-school improvement initiatives. Forty-two methodologically diverse empirical studies were used to inquire about department and department-head contributions to secondary-school improvement. Results indicate that department and department-head effects on students are consistently positive, practically meaningful and larger than school effects. Well-functioning departments are powerful centers for improvement. But significant hurdles to effective department-head leadership often minimize its effect, for example, some secondary teacher cultures, some teacher union policies, and some heads' own conceptions of their roles and responsibilities. Conditions enabling successful department-head leadership are identified.