To provide the most effective instruction possible, teachers of a second language (L2) should learn to identify and comprehend significant individual differences in their students. Many excellent teachers have learned to do some of this intuitively, but explicit understanding of individual-difference dimensions can enhance the work of all teachers. Among the most important such variations are differences associated with nine factors: aptitude, motivation, anxiety, selfesteem, tolerance of ambiguity, risk-taking, language learning styles, age, and gender. All of these variables and many more have been shown to be related to L2 learning in various ways. In fact, Gradman and Hanania (1991) identify 22 individual-difference variables that can affect success in learning a new language.