Introduction

The Second Language Acquisition (SLA) field began as an interdisciplinary endeavor in the broader field of Applied Linguistics over half a century ago, with its early research efforts drawing on intellectual developments on language and learning from the fields of linguistics and psychology. The approaches to SLA represented in the transdisciplinary framework include the biocultural perspective, complexity and dynamic systems theory, conversation analysis, language socialization, social identity theory, the sociocognitive approach, sociocultural theory, systemic functional linguistics, usage-based approaches, and variationist sociolinguistics. The chapter then summarizes the multifaceted, dynamic nature of SLA. It derives eight fundamental themes about the nature of language and learning from the three interdependent levels of social activity. Each theme offers action possibilities for L2 research, L2 learning, and L2 teaching.