The importance of theory in guiding research and development efforts in education is discussed. Issues related to academic foundations, professional education, and teaching fields that need to be addressed when developing a theory of teacher education include: (1) what effect different amounts, sequences, and combinations of content will have on the achievement of specified purposes of education; (2) what effect different delivery methods or experiences (e.g., lecture, laboratory, seminar, field experiences, simulations) and different student processing modes (e.g., reading, writing term papers, memorizing, observing, practicing, discussing ideas in small groups) will have on the achievement of the purposes of teacher education; and (3) what effect allocating different content, delivery methods, and student response modes to different stages in the career development of the teacher will have on the achievement of purposes of teacher education. (JD)