This chapter provides an in-depth discussion of the theory, research, and practice of individual difference (ID) factors in task-based language teaching (TBLT). It starts with an overview of the scope and taxonomy of IDs in second language learning, followed by a discussion of the theoretical models that provide insights into IDs’ roles in TBLT. It then introduces an organizing framework covering: (a) the nature of task IDs, (b) the utility of implicit IDs in TBLT, (c) the dynamicity of task IDs, (d) the expanded scope of task IDs, and (e) the importance of research on ID-treatment interaction. Subsequently, the chapter shifts its focus to the research on IDs in TBLT, including methods, perspectives, and empirical evidence on major ID factors – emotions (anxiety and enjoyment), language aptitude, motivation, and working memory. Based on the research findings, recommendations are made on ways to apply research findings to pedagogy from five perspectives: changing ID propensities, adapting instruction to mitigate or cancel ID effects, matching ID profile and instruction type, using hybrid instruction, and providing choice. The chapter concludes by providing an overview of the significance and contributions of this volume and highlighting prominent findings of the included chapters.