In the past few decades, public schools in the United States have witnessed a fast-growing student population of English language learners (ELLs), who come from homes where a language other than English is dominant. The dual task of content and the English language learning has brought ELLs huge academic challenges, such as one-size-fit-all expectations, high-stakes tests, and unsatisfactory academic support, among many others. In addition, ELLs are often mainstreamed into content area classes where teachers are underprepared to accommodate their learning needs. Thus, how mainstream teachers deal with ELLs for their content and English language learning merits further study. Educational scholars have paid increasing attention to the construct of teacher belief about teaching and learning. So far, studies on teachers' beliefs suggest that the complex relationship between teacher beliefs and practice are not always static but rather emergent and likely to be moderated by contextual factors (e.g. Negueruela-Azarola, 2011). As Priestley, Biesta, and Robinson (2015) argued, examination of the issue should recognize the immediate situated context and focus on the beliefs-in-action to probe why the teacher makes decisions in that particular moment and for what purposes. Learning, from a sociocultural perspective, is achieved through the interactions between the teacher and students, as well as among the students themselves. In the classroom, teachers usually dominate the flow of the discourse and their beliefs on teaching and learning, to some extent, can shape the way of such teacher-student interaction and students' learning. Classroom interaction is highly contextualized, spontaneous, and out of expectations. How the teacher manages this interactive practice and provides mediated support toward ELLs for both their content and language learning warrants further attention. This study draws upon a sociocultural perspective on learning, and a perspective that teachers' decision-making is fluid, situated and context dependent. The present study examines how a mainstream teacher in a U.S. urban school manages her interaction with ELLs to scaffold their English and content learning and how these practices reflect her beliefs-in-action. This case study, using data generated from stimulated recall interviews, classroom videos, and observation notes, reports this teacher's discursive practices with ELLs, along with her ongoing decision-making or her beliefs-in-action in this interactive process. The findings revealed that in this highly structured and teacher-dominated class, the interactive practice between the teacher and ELLs were limited. The teacher's feedback on ELLs' responses was not based on their learning needs but was impacted by some other contextual factors. The teacher's strategies to scaffold ELLs' learning were not as effective as she might expect: some are supportive of their learning, while others could impede their learning. Overall, the teacher's challenges and dilemmas that arise from her interactive practice with ELLs were largely due to her lack of formal training in second language teaching. This study sheds light on the complexity of mainstream teachers' beliefs and practices about ELLs. Furthermore, it advances our thinking on teachers' practices and beliefs by bridging the link between teacher beliefs and classroom interaction in an interactive, moment-by-moment manner through the fine-grained analysis. In addition, it offers implications on better supporting and preparing mainstream teachers working with ELLs in a culturally and linguistically diverse environment. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]