In this paper we review current thinking on the role of the head of department in secondary schools which emphasises that theirs is the critical agency in school improvement. We go on to describe a two‐fold strategy for developing successful schools which requires departments to improve: their teaching and their pupils’ learning; their capacity to both make and implement policy and so to facilitate the progress of change. We suggest that improving teaching and learning is best addressed at the departmental level by an evolutionary approach that emphasises vision, commitment, planning, action and review, rather than grandiose aims, statements, over elaborate policies and detailed, long‐term plans. In addition, we suggest that a department’s capacity to implement change depends on seven essential processes but that the leadership of heads of department is the key to developing successful departments and successful schools.