This book suggests that applied linguistics research is inherently concerned with complexity, emergence and causality, and because of this it also requires a robust social ontology. The book identifies and unpacks a range of conceptual issues in applied linguistics from a social realist perspective, and provides a critique of successionism and interpretivism as two dominant and enduring empiricist tendencies in the field. From this critique, it considers the emergence of complex dynamic system theory as viable yet not entirely unproblematic conceptual sophistication of current applied linguistics research. Although the growing popularity of complex dynamic system theory is undeniable and understandable, this book argues that its integration within a social realist ontology is necessary for further developments in the field. The book will be of interest to applied linguists and social scientists interested in language-related issues including language learning and teaching, language change, language policy and planning, bilingualism/multilingualism, and language and identity.