The last decade has seen a growing body of research investigating various aspects of L2 learners’ performance of tasks. This book focuses on one task implementation variable: planning. It considers theories of how opportunities to plan a task affect performance and tests claims derived from these theories in a series of empirical studies. The book examines different types of planning (i.e. task rehearsal, pre-task planning and within-task planning), addressing both what learners do when they plan and the effects of the different types of planning on L2 production. The choice of planning as the variable for investigation in this book is motivated both by its importance for current theorizing about L2 acquisition (in particular with regard to cognitive theories that view acquisition in terms of information processing) and its utility to language teachers and language testers, for unlike many other constructs in SLA ‘planning’ lends itself to external manipulation. The study of planning, then, provides a suitable forum for demonstrating the interconnectedness of theory, research and pedagogy in SLA.