Second language learners differ in how successfully they adapt to, and profit from, instruction. This book aims to show that adaptation to L2 instruction, and subsequent L2 learning, is a result of the interaction between learner characteristics and learning contexts. Describing and explaining these interactions is fundamentally important to theories of instructed SLA, and for effective L2 pedagogy. This collection is the first to explore this important issue in contemporary task-based, immersion, and communicative pedagogic settings. In the first section, leading experts in individual differences research describe recent advances in theories of intelligence, L2 aptitude, motivation, anxiety and emotion, and the relationship of native language abilities to L2 learning. In the second section, these theoretical insights are applied to empirical studies of individual differences-treatment interactions in classroom learning, experimental studies of the effects of focus on form and incidental learning, and studies of naturalistic versus instructed SLA.