While the second language (L2) teacher cognition literature has extensively documented what language teachers think, know, believe, and do (cf. Borg, 2006), far less is known about the dynamics of L2 teacher development arising from engagement with L2 teacher education pedagogies. Addressing this problem, Kubanyiova and Feryok (2015) contend that the field has yet to elaborate a coherent account of the development of L2 teacher expertise and the role of teacher education in systematically and intentionally leading such development. For Kubanyiova and Feryok (2015), two key questions at the core of the L2 teacher cognition enterprise, "[h]ow do language teachers create meaningful learning environments for their students" and "[h]ow can teacher education, continuing professional development, and the wider sociocultural context facilitate such learning in language teachers" (p.435) remain inadequately addressed and ripe for further inquiry. Directly addressing these issues, the present dissertation examines the development of six student-teachers in a Teaching English as a Second Language course at a large mid-Atlantic university as they engaged with and were shaped by a teacher education pedagogical intervention. Grounded in Vygotskian sociocultural theory (Vygotsky, 1987) and concept-based language instruction (C-BLI) (Lantolf, Xi, & Minakova, 2021), the praxis-oriented intervention both documented and provoked the student-teachers' development of a "teaching as dialogic mediation instructional stance" (Johnson, 2009). Encapsulated in a select coherent set of "pedagogical concepts" (Johnson, Verity, & Childs, 2023), a teaching as dialogic mediation instructional stance compels L2 teachers to consider the quality and character of their instruction, the relations between their teaching and student engagement, the importance of offering assistance and guidance that is highly responsive to learners' immediate needs and goals, and the reasoning driving their instructional decision-making. Across a series of individual and collective case studies, the results demonstrate the power of the pedagogical concepts embedded in the C-BLI intervention in how the student-teachers took up and productively used these psychological tools to confront the challenges of learning to teach ESL. Also vital to understanding the nature and trajectory of student-teachers' development were the quality and character of the teacher educators' mediation. The forms of contingent, dynamic, and responsive mediation offered by the teacher educators were essential in leading the student-teachers to confront the developmental dramas they faced in the course and to develop more sophisticated orienting bases of action toward L2 instruction. Concomitantly, the results of the study call into focus the importance of attending to the sociohistorical basis for language teacher development. To make sense of how the student-teacher participants developed, it was critical to consider prior teaching/learning experiences as the prisms through which they interpreted and attempted to take up the praxis-oriented pedagogy to which they were exposed. More broadly, this dissertation represents a concerted, theorized effort to show that high-quality L2 teacher education pedagogy "matters" for the development of L2 teacher expertise. [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.]