This paper examines the fundamental yet largely neglected distinction between school subjects and academic disciplines in the discourse on teachers' specialized subject-matter knowledge. It analyses and critiques the curricular positions embedded in that discourse in the light of five possible relationships between school subjects and academic disciplines. Invoking Dewey's logical-psychological distinction and research findings, the paper argues that the subject matter of a secondary-school science subject instead of the subject matter of its parent academic discipline lies at the heart of secondary-school science teachers' specialized subject-matter knowledge. Knowing such subject matter entails knowing five intersecting dimensions: the logical, the psychological, the pedagogical, the epistemological, and the sociocultural. Implications are drawn concerning theorizing about what secondary-school science teachers need to know about the subject matter they are expected to teach, subject-matter preparation for teaching, and the development of secondary-school science curriculum materials. (Contains 2 tables and 22 notes.)