This Element aims to elucidate the concept of language teacher agency by exploring the 'what' question, offering major conceptualisations of agency and explaining how they shape the way we approach teacher agency. The authors then continue with the 'why' question, and elaborate on the reasons that language teacher agency matters, based on a discussion of the varied purposes of teacher agency at multiple levels. They also acknowledge that teacher agency does not operate alone, and discuss how it intersects with such concepts as teacher identity, emotion, belief and knowledge. Based on this, they identify ways to promote teacher agency through making changes to contexts and/or actors. They then introduce the concept of collective agency and propose a multi-layered model based on an illustrative study. The Element ends with a call for a trans-perspective on understanding language teacher agency so as to facilitate the professional development of language teachers.