The theme of this volume, complexity, accuracy and fluency (CAF) as dimensions of second language production, proficiency and development, represents a thriving area of research that addresses two general questions that are at the heart of many studies in second language acquisition and applied linguistics: What makes a second language (L2) learner a proficient language user? And how can L2 proficiency be most adequately (i.e. validly, reliably and feasibly) measured? Many L2 practitioners and SLA researchers, including the contributors to this volume, now hold that L2 proficiency is not a unitary construct but, rather, that it is multicomponential in nature, and that its principal components can be fruitfully captured by the notions of complexity, fluency and accuracy, or CAF for short. In recent years the CAF triad has emerged as a notable complement to other established proficiency models such as the traditional four-skills model and sociolinguistic and cognitive models of L2 proficiency. The status of CAF as principal and distinct dimensions of L2 performance and proficiency has now been justified both empirically and theoretically. Empirically, factor analyses have identified complexity, accuracy and fluency as distinct and competing areas of L2 performance, implying that all three must be considered if any general claims about learners' L2 performance and proficiency are to be made. Theoretically, these three dimensions have been claimed to imply the major stages of change in the underlying L2 system. In sum, from this diverse body of research, complexity, accuracy and fluency emerge as distinct components of L2 proficiency and performance, which may be differentially manifested under different conditions of L2 use, and which may be differentially developed by different types of learners and under different learning conditions. [For complete volume, see ED539539.]