The process of learning an additional language and its outcomes are affected by numerous factors. While some of these factors, such as access to the target language in the immediate environment or the teaching methods and materials used, are external to the learner, others, which comprise a wide array of individual differences, are learner-internal. The present chapter focuses on the latter set of factors which in most cases can trump the effects of any other influences. It first offers a brief overview of the evolution of research into individual difference (ID) variables that has taken place over the last several decades. It then provides an overview of selected ID factors, with special emphasis on five that are directly relevant to language pedagogy: motivation, willingness to communicate, beliefs, language learning strategies, and emotions. In each case, an attempt is made to discuss the ways in which these variables can be manipulated to enhance language learning. More general guidelines on how language teachers can deal with individual variation in their classrooms are also offered.