This paper addresses the question of what role the historical discipline might play in informing the selection of substantive knowledge for school history curricula. In the process, it seeks to clarify the usefulness and limitations of Young's social realist theory of powerful knowledge in the case of school history. The paper proposes that assessing the potential of the historical discipline for informing substantive knowledge selection in school history requires a more thorough account of the historical discipline's horizontal knowledge structure. Having attempted such an account, it is argued that while the historical discipline offers no consensus on exactly what substantive knowledge to teach, it does offer resources for tackling political and ethical questions inherent in substantive knowledge selection in school history. This is exemplified through the case of environmental history. The paper concludes that realizing the potential of the historical discipline to contribute to questions of substantive knowledge selection in school history requires that history educators move beyond theorizing the distinction between vertical and horizontal discourses, central to Young's theory of powerful knowledge, and, drawing on Bernstein, consider the historical discipline's particular horizontal knowledge structure and its dialogic, often critical, entanglement with horizontal discourses.