This textbook takes a Complex Systems Theory approach to examine individual differences between learners and the potential impact of these variables on the process of acquiring a second language. The authors argue that individual variables cannot provide the complete picture, and that they must instead be understood as part of an interconnected and dynamic system of different factors in order to be useful in a language learning context. Written in an accessible style and suitable for final-year undergraduate and Masters-level students, the book includes clear definitions of key terms, discussion questions for classroom use, practical exercises and activities, and examples of real empirical studies that students and teachers can replicate in their own contexts. This textbook will be of interest to students taking TESOL and SLA courses and modules, as well as those on broader Applied Linguistics programmes.