This article is broadly concerned with the differences between individual language learners. In terms of particular content areas of Individual Differences (ID) research, it surveys developments in foreign language aptitude, motivation, learner strategies, and learner styles. A brief review of earlier research on aptitude is presented, followed by discussions of more contemporary work on the origin of aptitude, namely, as a residue of first language learning ability, and on the existence of evidence for “learner types.” Motivation research is reviewed partly with regard to Robert Gardner's research, and then in terms of a wider framework for the functioning of motivation within an educational context. The review of learner strategies research emphasizes current attempts to develop taxonomies of such strategies, and to investigate their theoretical basis and their trainability. Finally, learner styles research, drawing on field independence theory, is discussed, and links are made with the research on aptitude. The article finishes with sections on conceptual and methodological issues in ID research.