This study concerns the issue of secondary science teachers' attention. In particular, I consider if, how, and when science teachers attend to the substance of student thinking, which is called for by science education reform (NRC, 2007). Using a case study approach, and drawing on ethnographic data sources, I explore what novice and experienced secondary science teachers regularly attend to while teaching, what shapes teachers' attention, and how teachers' attention is consequential for students' science learning. I find that both novice and experienced teachers can attend to the substance of student thinking, although the institutional and social systems of school draw teachers' attention to other foci--particularly to correctness of conceptual knowledge and the vocabulary that signals correctness and "misconceptions." Furthermore, I argue that when teachers regularly attend to the substance of student thinking, they can contribute to a classroom culture that supports student inquiry. I discuss implications of this work for understanding teaching and for teacher education and professional development, and I suggest areas for future research that are motivated by these findings.