This document summarizes several factors that influence individual differences in second language acquisition: language aptitude, motivation, and learning strategies. It describes John Carroll's theory of language aptitude consisting of phonemic coding ability, grammatical sensitivity, inductive language learning ability, and rote learning ability. It defines instrumental, integrative, resultative, and intrinsic motivation. It categorizes learning strategies as cognitive, metacognitive, or social/affective and notes that good language learners are very active, aware of their learning process and style, and flexible in using appropriate strategies.