There is increased attention in the science education community on the importance of engaging students in the practices of science. However, there is much to be learned about "how" students enter into and sustain their engagement in these practices. In this paper, we argue that "epistemic affect"--feelings and emotions experienced within science, such as the excitement of having a new idea or irritation at an inconsistency--is part of what instigates and stabilizes disciplinary engagement. We first discuss affect as evident in accounts of professionals; we then show its emergence and role in 2 case studies of elementary school students. In the end, we argue that epistemic affect is part of the "substance" that students should learn in science, and we discuss implications for research and instruction.