This study investigated the relationship between teachers' comprehension monitoring strategies instruction and students' reading comprehension in the context of the Interactive Strategies Approach--Extended intervention (ISA-X, Gelzheiser et al., 2011). In the ISA-X, the teachers were encouraged to provide third and fourth grade students with comprehension monitoring strategies instruction, which was called "Mix up? Fix up!" (MUFU) strategies instruction. The study focused on students who had grade-appropriate reading accuracy but limited reading comprehension. Mixed methods were used to examine the effects of teachers' instruction about the MUFU strategies on students' strategy knowledge and reading comprehension. For the quantitative research, eighty-five third and fourth grade students and eight teachers were included. Teachers' implementation fidelity was rated and students' fix-up strategies knowledge and reading comprehension were also assessed. Results showed that students made small gains in fix-up strategies knowledge and that there was no association between teachers' implementation fidelity and students' fix-up strategies knowledge. However, a significant association was found between teachers' implementation fidelity and students' reading comprehension at post-intervention. After controlling for grade level and pre-intervention reading comprehension, the association remained significant. In order to further understand how implementation fidelity impacted students' reading comprehension, I conducted qualitative analyses in which I observed two teachers and six students of each teacher. Based on the proportion of their students who made gains in reading comprehension, the two teachers were characterized as the typical teacher and the above average teacher. The goal was to examine whether there were certain patterns in the teachers' teaching behaviors that might have contributed to the students' reading comprehension gains. Using the coding system in the Instructional Quality Rubric-MUFU strategies, I coded the teachers' MUFU strategies instruction and the students' strategy learning in both the acquisition phase and the consolidation phase. Both teachers provided students with relatively few opportunities to learn about and practice the MUFU strategies. Both teachers were able to release some responsibility to students, but at the end of intervention students still needed teacher support to use the MUFU strategies. The two teachers showed different patterns in their MUFU strategies instruction, especially while modeling the MUFU strategies. In think aloud modeling, the above average teacher chose to solve confusions caused by words with unknown meaning, or sentences or passages where the meaning was confusing. In contrast, the typical teacher focused on confusions caused by puzzling words that could not be identified. Moreover, the typical teacher entangled word identification strategies and the MUFU strategies together, which could have been confusing to the students. I found similar patterns in students' MUFU strategies application during reading which suggests that the differences in the teachers' modeling were powerful and led to differences in student learning. Compared to the typical teacher, the above average teacher also demonstrated more responsiveness and flexibility in that she was able to allow the students to confirm their understanding when reading made sense, and to rectify the students' inappropriate reading practices. Thus, it seems that the above average teacher was more explicit and responsive in providing instruction on the MUFU strategies. Allahverdi's (2018) mixed methods study using the same samples found that the above average teacher was also more explicit in providing instruction on purpose setting, questioning, and predicting. I concluded that difference in the teachers' rate of student comprehension gains was likely due to differences in their general effectiveness rather than to their MUFU strategies instruction alone. The study had a few limitations. The strategy knowledge and fidelity measures did not provide full information. MUFU strategies instruction was brief, and teacher professional development was not sufficient for teachers to fully understand the strategies being taught. It would be useful to examine intensively-provided MUFU strategies instruction in future research. It would also be interesting to investigate the teacher-learning and teacher-teaching processes in the context of the MUFU strategies. [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.]