The volume begins by outlining the development of the teacherstudent relationship according to the main psychological theories. Next, it points to the inseparability of cognitive and emotional factors in the learning process. By referring to the major positive psychology theories, i.e., Broaden-and-Build, Flow, and PERMA, the book lays the foundation for studying the full spectrum of learner emotions experienced while learning a foreign language. Finally, it provides a research-based analysis of teacher-related variables that affect learner classroom well-being and ill-being.