This article is broadly concerned with the individual differences among language learners. In terms of particular content areas of Individual Differences (ID) research, it surveys developments in foreign language aptitude, motivation and affective factors, and interaction of learners' aptitudes and teaching methods which is called Aptitude Treatment Interaction (ATI). A brief review of theory of second language acquisition is presented, followed by discussions of research on aptitude. Motivation research is reviewed partly with regard to Gardner's research, followed by other researches including those in Japan. Finally, the review of ATI research are presented to emphasize attempts to investigate adaptive education to individual differences. The article concludes with a section on future issues in ID research in Japan.