This book argues that, in line with the tenets of positive psychology in SLA and the rhetorical/relational goal theory, positive teacher-student interpersonal relationships are deemed to be of great significance for empowering students to accomplish favorable academic outcomes and to successfully learn a second/foreign language (L2), whether at its affective, behavioral, or cognitive levels. Therefore, understanding the role of teacher interpersonal behaviors and their effect on students' learning gains in the domain of SLA is of utmost importance, particularly as this line of research is at its nascent stage of development, and, as a result, available empirical evidence is still inconclusive. To address this issue,  drawing on the mixed methods design, this book mainly aims to, first, empirically scrutinize the role of "5Cs" positive teacher interpersonal variables (i.e., care, clarity, closeness, confirmation, and credibility) in L2 students' affective, behavioral, and cognitive learning outcomes through the mediation of student-perceived learner empowerment in the L2 context of Iran. Second, it is intended to show how L2 teacher educators, teachers, and materials developers, among other key educational stakeholders, can facilitate the provision of interpersonally rich language learning environments with the ultimate goal of enhancing students' L2 learning.