Ever since the early days of its existence, the field of psychology has been trying to achieve two different and somewhat contradictory objectives: to understand the "general principles" of the human mind and to explore the "uniqueness" of the individual mind. The latter direction has formed an independent subdiscipline within the field, usually referred to as "individual difference" (ID) research. IDs are a prominent feature of SLA because a great deal of the variation in language learning outcomes is attributable, either directly or indirectly, to various learner characteristics. This paper first provides an overview of the five most important ID variables (personality, aptitude, motivation, learning styles and learning strategies) and then concludes by describing certain common themes in contemporary ID research.