This book highlights the roles of several individual difference (ID) variables on the language learning process, exploring them from both the students’ and the teachers’ perspectives. It presents the results of a large-scale, mixed-methods investigation which was conducted with secondary school pupils and their teachers in Hungary. The quantitative questionnaire data is used to analyze the English language learners’ motivation, autonomy and self-efficacy beliefs, and to examine the relationships between these and a wide range of positive and negative emotions. The qualitative data, consisting of interviews with teachers, gives voice not only to an understanding of student-related ID variables but also to teachers’ reflections on their own cognitive, affective and behavioral processes. Taken together, the contrastive analysis of these two datasets yields interactional results that provide fresh insights into the language learning process and practical classroom applications.