This book addresses a particularly important area of second language acquisition research, specifically the factors that mediate learning outcomes in learning a second or foreign language. It provides a comprehensive, up-to-date overview of studies in the field of second language learning and L2 use from the point of view of personality traits and emotional intelligence. It also presents results of a mixed-method study researching the possible influence of these variables on the process of learning a second language. Furthermore, the book represents the first systematic exploration of the role of emotional intelligence and the factors related to it in moderating attainment in the target language. What is also unique here is the investigation of the role of personality with respect to different target language skills and subsystems, which allows finer-grained insights into this relationship. The book will be of considerable interest to both researchers, teachers and students in the fields of SLA, TESOL, applied linguistics, and bilingualism.