The issue of science subject knowledge, and how to address the demands of this for both practising and trainee primary teachers, has constituted a core research enterprise in recent decades. The Professional Standards for Qualified Teacher Status entail considerable conceptual demands for many primary trainees. Generating meaningful causal explanations of scientific phenomena lies at the heart of both the scientific endeavour itself, and of effective classroom teaching. To focus on knowledge acquisition per se in teacher education, however, obscures the critical issue of subject-related pedagogical knowledge that ultimately influences classroom practice. This article explores the development of both subject knowledge and subject-related pedagogical knowledge in science education. It is informed by science education literature, as well as by a substantial body of empirical research into trainees' learning of aspects of physical science accumulated over a five-year period. Learners' perspectives of the synthesis of subject and pedagogy raise important questions concerning the nature of teacher education.