Against a background which recognises pedagogical content knowledge as the distinctive element of teacher competence/expertise, this theoretical essay argues for its central construct--that of transformation--to be understood by teachers and teacher educators in psychological terms (as was originally proposed by Dewey). Transformation requires teachers to fashion disciplinary knowledge such that it is accessible to the learner. It is argued that for transformation to happen, teacher thinking must include a sophisticated grasp of cognition and metacognition if teachers are to be characterised as competent, let alone expert. This article is written within a context of considerable social and academic scrutiny in the UK of the form and content of professional teacher preparation and development. In recent years, the contribution of psychological knowledge to teacher education has been filtered through procedural lenses of how best to "manage classrooms", "assess learning", "build confidence" or whatever without a matched concern for psychological constructs through which such issues might be interpreted; thus, leaving teachers vulnerable in their professional understandings of learning and its complexities. That society now requires high-level cognitive engagement amongst its participants places cognitive and metacognitive demands on teachers which can only be met if they themselves are conceptually equipped. (Contains 1 table.)