Promoting facility with content knowledge is one of the most important objectives of science teaching. Conventionally, the focus for this objective is placed on the substantive side of content knowledge (e.g. science concepts/laws), whereas its epistemic or ontological aspects (e.g. why do we construct concepts?) rarely receive explicit attention. In this article, we develop a theoretical argument for the value of elevating the attention paid to the epistemic/ontological aspects of content knowledge and integrating them with its substantive side. Our argument is structured in two parts. The first unpacks the epistemic/ontological aspects of content knowledge and their role in science. For this, we focus on two specific aspects (i.e. ontological status and epistemic value of science concepts), which we elaborate in the context of two particular content domains, namely magnetism and energy. The second part of the argument highlights the potential of discourse on epistemic/ontological aspects to facilitate learning in science. We delineate how such discourse could (a) promote coherent conceptual understanding, (b) foster a productive epistemological stance towards science learning, and (c) enhance students' appreciation of ideas associated with the nature of science. The article concludes with a discussion of ensuing implications for science education.