This volume examines the agency of second/foreign language teachers in diverse geographical contexts and in both K-12 and adult education. It offers new understandings and conceptualizations of second/foreign language teacher agency through a variety of types of empirical data. It also demonstrates the use of different methodologies or analytic tools to study the multidimensional, dynamic and complex nature of second/foreign language teacher agency. The chapters draw on a range of theories and approaches to language teacher agency (including ecological theory, positioning theory, complexity theory and actor-network theory) that expand our understanding of the concept, while at the same time presenting various analytic approaches such as discourse studies and narrative inquiry. The chapters also analyze the connection of agency to other relevant topics, such as teacher identity, emotions, positioning and autonomy.