This book presents comprehensive, thorough and updated analyses of key cognitive individual difference factors (e.g., age, intelligence, language aptitude, working memory, metacognition, learning strategies, and anxiety) as they relate to the acquisition, processing, assessment, and pedagogy of second or foreign languages. Critical reviews and in-depth research syntheses of these pivotal cognitive learner factors are put into historical and broader contexts, drawing upon the multiple authors' extensive research experience, penetrating insights and unique perspectives spanning applied linguistics, teacher training, educational psychology, and cognitive science. The carefully crafted chapters provide essential course readings and valuable references for seasoned researchers and aspiring postgraduate students in the broad fields of instructed second language acquisition, foreign language training, teacher education, language pedagogy, educational psychology, and cognitive development.