Chapter 1 locates the discussion of teacher agency in the wider theoretical propositions about human agency as a capacity make a difference in the world. It reviews some of the most influential theories of agency including sociological approaches such as Giddens’ structuration theory and Archer’s critical realist view of agency, MacIntyre’s virtue ethical philosophy, psychological perspectives of Bandura, as well as relational theories proposed by Emirbayer and Miche that reflect in ecological approaches to consider their applications to teachers’ work specifically. The chapter also outlines how these theoretical perspectives have informed the organisation of the book chapters that present relevant research and discus its implications for teacher development.