Since the introduction of task-based language pedagogy in the 1980s, tasks have held a central place in current second language acquisition research and pedagogy. The assumption that in a task-based syllabus, pedagogic tasks should be sequenced to increasingly approximate the demands of real-world target tasks (Robinson, 2001a, 2001b) has given rise to the concept of “task complexity”. In other words, the concept of task complexity springs from the need to establish criteria for sequencing tasks in a syllabus from simple to complex in a reasoned way that will foster interlanguage development. The present book operationalised the well-established “Triadic Componential Framework” model, also known as “Cognition Hypothesis” by Robinson (2001a, 2006) to explore the effect of task complexity manipulation and language proficiency levels and their possible interaction on L2 writing task performance. The content of this book can be especially interesting to second language acquisition researchers and practitioners working within the domain of task-based language teaching, task-based syllabus design and materials development.