This research examines the specialized knowledge of reading comprehension articulated by 12 middle school English language arts teachers sampled from three different regions of the United States and representing various levels of teaching experience. Using detailed interviews, concept mapping, and think aloud methods, we identified salient touchstones that characterize the teachers' understandings of how and why readers interact with texts to construct meanings and change as readers. In this article, we present examples that illustrate the many ways that these understandings of comprehension were mobilized and transformed in their explanations of instructional practices.