Using emergent motivation theory, this study analyzes the relationships between high school students' perceptions of their reading skills, task challenge, and personal control over assigned reading activities and their reading engagement (i.e., interest, enjoyment, and concentration) during science instruction. The study also examines how these relationships differ between struggling and proficient readers. Further, the study examines the association between students' experiences of conditions of anxiety, apathy, boredom or relaxation and flow on their reading engagement during science instruction. Finally, the study investigates the relationship between reading engagement and science achievement. The experience sampling method (ESM) was used with a sample of 244 high school students in grades 9-12 to measure students' perceptions of skill, challenge, and control, and their reading engagement during science instruction. Hierarchical linear modeling (HLM) was employed with repeated-measures analysis of students' momentary experiences of skill, challenge, control, and engagement during reading. Results show that when students perceive having both high reading skills and high control, they exhibit deeper reading engagement than when they perceive having both low reading skills and low control. These feelings of control were noticeably important for struggling readers as compared to proficient readers. It was also found that students experience deeper reading engagement during flow (i.e., when students perceive they have high reading skills and the reading task is highly challenging), but reading engagement decreases significantly whenever students experience conditions of anxiety or apathy when reading science materials. Finally, reading engagement was positively related to science achievement. Educational implications of the results are presented. Limitations of the study are discussed and suggestions for future research are described. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]