Moment-to-moment emotions are affective states that dynamically change during reading and potentially influence comprehension. Researchers have recently identified these emotions and the emotion trajectories in reading, tutoring, and problem solving. The primary learning-centered emotions are boredom, frustration, confusion, flow (engagement), delight, surprise, and anxiety. Emotion transitions occur when the text becomes too difficult or easy for the reader and when conceptual obstacles create cognitive disequilibrium. Teachers and computer environments have the potential to improve reading comprehension by detecting and strategically handling the readers' boredom, frustration, and confusion. One frontier in reading research is to understand the complex dance between comprehension and emotions.