Effective teacher supervision for successful schools seeks to remove obstacles in the work environment that hinder teachers from seeing each other at work, receiving feedback from others, engaging in professional dialog, and participating in decisions about collective instructional actions. Just as teachers try to encourage the cognitive development of students, so supervisors should try to develop the thinking abilities of teachers. Supervisory tasks that can be planned to provide teachers with these developmental opportunities include direct one-on-one assistance to teachers, inservice education for groups of teachers, curriculum development activities, and action research. Adopting uniform supervisory and teaching practices fails to allow for or to encourage the mediating effects of thought. This paper reviews the role of the supervisor (that is, of the many people involved in supervision), the importance of the supervisor's activities as models for the teacher's, the characteristics of different types of conceptual thinking, and the difficulties (often growing out of the historical development of American education) that beset educational improvement efforts. The central focus of the paper is on the nature of teacher development strategies that supervisors can employ. A list of 38 relevant resources is attached to the document. (PGD)